216

CARBONIFEROUS CALCAREOUSM/TCHELDEAN/A WETHERED ANDGARWOOD/A, GEN. NOV.

THE LOWERALGAE

By ALAN WOOD, Ph.D., F.G.S.

[Received 7th March, 1941.]

HISTORICAL REVIEW.MITCHELDEANIA, with type-species M. nicholsoni, was

first described by Wethered (1886) from the LowerLimestone Shales (K zone) of the Forest of Dean. It wasconcluded that Mitcheldeania was" an organism the structure ofwhich consisted of a series of concentrically arranged layerspenetrated by two systems of tubuli, the larger measuring .003and the smaller .001 of an inch in diameter.' The latter wereprobably filled with living matter and the larger I regard aszooidal tubes." Another, more minute, system of tubes wasseen, considered to be a canal system penetrating" the skeletonfibre." The presence of the two larger systems of tubuli sug­gested that the genus was a hydrozoan, and it was placed in theHydractinidae. Unfortunately the figure accompanying thepaper was unrecognizable, but many figures were given in a latercommunication (1887). Shortly afterwards Nicholson (1888)redefined the genus Mitcheldeania, with the help of " a muchlarger species" (M. gregaria) from the Lower Carboniferousrocks of the South of Scotland. Again two sets of different sizedtubuli were found, and the concentric layered structure was seen,but emphasis was now laid on the radiating character of thetubuli and mural pores were stated to occur. This new defini­tion was taken wholly from the species M. gregaria. Wetheredhimself (1889) accepted M. gregaria as congeneric with M.nicholsoni. Some twenty years later Professor Garwood (1913,1914) placed Mitcheldeania in the calcareous algae, and showedthat the mural pores described by Nicholson were in realityeither elbows in the undulating tubes or places where a branchwas given off at right angles to the plane of the section. In 1931the same author showed that the finer tubes seen by Nicholsonin M. gregaria belonged to a new species of the genus Ortonella.The specific name gregaria was thus restricted to the larger.. zooidal " tubes of Nicholson. Finally, Pia (1937) in a masterlysurvey of the Upper Palaeozoic algae commented on the obscurityof M. nicholsoni. In his definition of M itcheldeania (taken ofnecessity from M. gregaria) Pia drew attention to the peculiarmode of branching and contrasted it with Ortonella. A new sub-

I .076 and .025 mm,

LOWER CARBONIFEROUS CALCAREOUS ALGAE. ZI7

family Mitcheldeanieae was established, to include Mitcheldeania,Ortonella and H edstromia.

Type Specimen and Type-Locality of MitcheldeanlaNicholsoni.

rive of Wethered's original slides have been traced. Oneis in the Natural History Museum collection (Slide P. 448z) andfour in the collection at Bristol University (W. 4, W. ro, W. II,W. 14). The slide in the Natural History Museum was pre­sented by Wethered, and is labelled" Lower Limestone Shales,MitcheJdean, Gloucestershire." Of the specimens in the BristolUniversity two are unlocalized, and the labels of the other tworead "Wadley's Quarry,' bed No.8, near Drybrook, Forestof Dean, L.L.S." and" L.L.S. Brain's Quarry No. Z near Dry­brook, Forest of Dean." Neither of the algal nodules figuredby Wethered (1886 and 1887) occurs in these slides, but some ofthe drawings illustrating the 1887 paper may have been takenfrom them. A slide in the Judd collection, Imperial College,is labeJJed " Mitcheldeania Limestone, L.L.S., Drybrook, Forestof Dean," in the same handwriting as the authenticated Wetheredslides, and was probably presented to Judd by Wethered. Allthese slides have been used in the general description whichfollows.

The original specimens do not come from the well-knownalgal development in the Whitehead Limestone (Cz}, but fromthe Lower Limestone Shales of K age, and the original localitylies just east of the Deep Cutting on the west side of the Wigpoolsyncline-thus nearer Drybrook than Mitcheldean. There aretraces of much old quarrying on a small scale in this vicinity,and it does not appear safe to pick on anyone spot as the typelocality, though this almost certainly lies within zoo yards ofthe road S.W. of Silverton Farm (Sibly and Reynolds 1937,map, p. 24). Search in the grounds of Euroclydon, a house lyingon the south side of this road, yielded a solitary block of alga!limestone, lying loose among the debris on the ground, whichyielded thin sections of Mitcheldeania comparable to, but notidentical with, those seen in Wethered's slides.

Enquiry of the Postmaster of Drybrook reveals that a Mr.T. B. Brain lived at Euroclvdon itself. The reference to Brain'sQuarry on Wethered's slide may refer to some small quarryin the grounds of this house, though ML Brain owned at leastone other quarry, in the Drybrook Sandstone near Drybrook.

We know from Wethered's 1886 paper that the type ofMitcheldeania figured therein came from the second of two

1 Dr. Stubblefield ill forms me that the quarry marked 'I Drybrook Quarries II on Stbtyand H.CYIloJ~ls (r 937) map was once owned by a 10(a1 farmer lli.IIl.;eJ \V;~d[ey, but since this quarryis in the Main Limestone and wethcrcd's slide is marked " L.L,:'." p(QlJ~bly two or more quarrieshad this name at different times.

218 A. WOO]),

adjacent quarries, and the" Brain's Qy. No. 2 " on the label ofthis slide suggests that the specimen may be from the originaltype locality. For this reason one of the algae in this slide islater selected as the neotype of M itcheldeania.

Redescription of Mitcheldeania Wethered.Small nodular bodies with a diameter of some 3 mm. occur in

these slides (Plate 13, Fig. I). Some of them are closely com­parable with Wethered's 1887 figure, and all the constituentparts can be distinguished. The larger tubules (zooidal tubes ofWethered) run in general parallel to the outer surface of thenodule, penetrating and twining among areas made up of smallertubuli of two sizes. One set of tubuli, the canal system ofWethered, is extremely minute, occurs in radially directed tuftsand branches dichotomously (PI. 15, Fig. 4), while the ot her setpursues an irregularly radial path and often makes up the bulkof the nodule.

Careful study of these three types of algal tubes, which arcvery irregular in their distribution even in the same nodule,reveals that they are the remains of three different algae at least.The largest tubuli belong to the genus Girvanella, the smallestto Ortonella, while the medium sized tubuli resemble the genusBevocastria. "Under these conditions it appears that a laterreviser has the power to choose which of the genera included insuch a composite genus shall be regarded as typical. It isextremely unfortunate that no tubes occur which can be regardedas congeneric with the large tubes of " j;[itcheldeania " gregariaNicholson since the interpretation of Mitcheldeania has beenbased on Nicholson's species for the past 50 years.

Of the different algae included in these composite nodulesonly the Girvanella and Ortcnella types can be definitely matchedwith Wethered's figures. These are unrnistakeable, but themedium-sized tubes, which may be compared with Bevocastria,are difficult to interpret. They are irregular in direction andin branching, while anyone section may show several differentaspects. It is probable that either different genera of thesemedium-sized tubuli are mingled together, or that the samespecies assumes widely different forms under the influence ofexternal conditions. To choose anyone of these medium-sizedtubuli as typical of Mitcheldeania would be extremely dangerous,for later work might show that the particular form chosen didnot occur in the original type-specimen of M. nicholsoni. Alsoit is advisable to leave later workers as free a hand as possiblein the naming of the extremely difficult and confused Bevocastria­like forms.

Since of the two remaining genera Ortonella is well known,easily recognizable, and includes a number of species any change

LOWER CARBONIFEROUS CALCAREOUS ALGAE. 219

in nomenclature would cause considerable confusion. It isdeemed advisable therefore to select as typical the largest (zooidal)tubes of \Vethered. Mitcheldeania Wethered then becomes asynonym of Girvanella Nich. and Eth.

A redescription of Girvanella nicholsoni (Weth.) IS necesary,since this form has been only cursorily described.

GIRVANELLA [MITCHELDEANIAJ NICHOLSONI(WETHERED).

(Plate 13, Figs. I and 2.)

Moulds of algal threads occurring in nodules, or as occasionaltwisted threads in limestones. A general concentric arrange­ment of the tubes is seen when the algae occur in nodules,while the more or less isolated tubes in limestones tend to liealong the bedding planes. Diameter of tubes ranging from.08 down to .04 mm. measured on cross sections' and checkedby observation of longitudinal sections. Curving of the threadsis less strong than in the other species of Girvanella. Branching(PI. IS, Fig. 3) apparently rather irregularly dichotomous,possibly polychotornous, with the branches diverging at ratherwide angles.

I have not been able to find any break in the range of sizeof these threads which might indicate the presence of twospecies.

REMARKS. Certain of these tubes imbedded in clear calciteshow a dark calcareous sheath (PI. IS, Fig. 2) closely comparablewith that seen in the original figure of Girvanella problematicaNich. and Eth., the genotype of Girvanella. The curvature of thetubes, and their mode of occurrence are closely similar, andthough the diameter of the tubes is from 3-5 times that seenin the genotype, there is no valid reason for separating thisspecies from the genus Girvanella. The forms of modern algae,however, are protean, and there are very few characters visiblein these. fossil forms so that it is improbable that the presentspecies and G. problematica are closely related. The nameGirvanella can only be used as a "form-genus."

The calcareous sheath which forms the wall of the tubes incertain cases (Fig. 2) is interpreted as precipitated calcite caughtin a mucilaginous coating of the algal thread, and not as a calcifiedcell wall. The clear calcite infilling is the size of the originalthread and the measurements given do not include the dark

I When the diameter of algal tubes is measured under the microscope error may creep inif cross sections are not measured. The curvature of the" wall" of the tube, which is alwaysmade up of rather opaque material, tends to render the apparent width of the tube less than thereal width, since only the transparent central portion of the tube can be dearly SCt'Il. The errormay become grccit (up to 50%) when the diameter of the tube is small in relation to the thicknessof the slide.

220 A. WOOD,

" wall" of the tube. Occasionally the algal threads wereimbedded in a thicker calcite precipitate which filled the wholespace between them and is now represented by opaque materialseparating the tubes. This is indistinguishable from thematerial composing the wall, and is exactly similar to thatseparating the clear calcite tubes of Bevocastria, Ortonella andother lime-precipitating genera. The term algal dust has beenused for this precipitated material (Wood, 1941).

There is no possibility that G. nicholsoni was a boringcreature, for the tubes are sometimes entangled in the finethreads of Ortonella , which are seen to bend round them (PI. 15,Fig. 4). Also the species occurs embedded in thin layers ofprecipitate around shells, with an absence of other algal tubes,just as do undoubted calcareous algae.

DISTINCTION FROM OTHER SPECIES. The present speciescan hardly be confused with any other British Lower Carboni­ferous Girvanella so far described. The tubes of G. ducii Weth.are only half as wide as the smallest tubes of G. nicholsoni,while those of G. wetheredii Chapman,' G. staminea Garwoodand G. ottonosia Pia are even more tenuous.

The only extra-British species of Girvanella comparable withG. nicholsoni was recently described by Paul (? 1937) asG. silesiaca from a calcareous band in the Culm of easternGermany (Dibunophyllum zone). The width of the tubes isstated to range from 55 to 65 J.L (the measurements probablyincluding the walls of the tubes). Direct measurement on thefigure given seems to indicate a range greater than this, and,assuming that the measurements are even approximatelycorrect, the magnification of the published figure is incorrectlygiven. Since the dimensions given fall approximately midwayin the range of variation of G. nicholsoni, and the figure showsa close resemblance to it in general aspect of the tubes, G.silesiaca is probably identical with the present species.

LOCALITY. See p. 217.TYPE SPECIMEN. Tube shown on PI. 13, Fig. 2, which

occurs in slide W 10 Bristol University collection, labelled inWethered's handwriting "Carboniferous Limestone. L.L.S.Brain's Quarry NO.2. Near Drybrook. Forest of Dean."Now in the Geological Survey Collection (No. 70740).

HORIZON. Probably from the nodular limestone at thebase of division 4 of the Lower Limestone Shales (Sibly andReynolds, 1937). Specimens of this band preserved in theGeological Survey Museum (sliced by courtesy of Dr. C. J.Stubblefield) show nodules containing a similar association of

I Chapman proposed this name in 1907 for G. incrustans Wet.hered non Bornemann. It isequiv.rlent to C. staminca Pia (? non Garwood).

LOWER CARBONIFEROUS CALCAREOUS ALGAE. 221

algae to those in Wethereds slides. It is unlikely that suchassociations of three genera would have a long range sinceprobably rather special conditions were needed to suit all ofthem, and this horizon is probably approximately correct.In any case the type specimens must come from the K zone.

This species is also found in the Km beds of the Avon section.Sections of the Mitcheldeania limestone figured by ProfessorReynolds (1921, Plate VIII, Fig. I) show G. nicholsoni in thesame slide as, but not growing entangled in, rather stuntedalgae probably belonging to the new genus Garwoodia (see later).

Other specimens occur in the Geological Survey collectionsfrom the Bewcastle district (ex-Garwood collection). Theywere collected from the Main Algal Series of Cr-C2 age at" Crubinshill " and" The Beck near 1,000 ft. contour" (G.S.M.slides 68rr6 and 69027). G. nicholsoni therefore has a relativelylong range in time, especially should G. silesiaca Paul be provedconspecific with it, since that species is recorded from theD zone.

Since no trace of tubes of the type of "Miicheldeania "gregaria J';icholson is found in the original M itcheldeania nodulesa new name must be found for them. It is proposed to callthis new genus Garwoodia in recognition of Professor Garwood'sadmirable work on the fossil calcareous algae.

Apparently Nicholson's types are lost, for search in themuseum of Aberdeen University has failed to reveal them.In any case it would be practically impossible to identify thetype specimens with certainty from Nicholson's line drawings.One slide of " M itcheldeania " gregoria from the type localityis preserved in the Wethered collection, Bristol University(Slide W. 13) and was almost certainly presented to Wetheredby Nicholson himself. In addition specimens of gregorialimestone from Kershopefoot, Roxburghshire, the locality whenceNicholson obtained his specimens, have been borrowed fromthe Geological Survey of Scotland by courtesy of Dr. Pringle,who informs me that the quarry at Kershopefoot is now flooded.It is proposed to select as Neotype the specimen in the Wetheredcollection.

GARWOODIA, GEN. NOV. ( = MITCHfLDEANIAAUCTORUM).

(PI. 14, Figs. I and 2.)Moulds of algal threads occurring in nodules and having a

markedly radial direction. The threads were practicallystraight for relatively long distances, so that a very characteristic

222 A. WOOD,

appearance is seen in section. Branching occurred ratherfrequently. The new thread was given off almost at rightangles from the parent, and it then bent round rapidly to growparallel to the other threads (Fig. 2). All threads close together,sub-parallel, and sub-equal in diameter.

DISTINCTION FROM OTHER GE~ERA. Differs from Ortonellain the mode of branching and in the closer packing of the tubes,from H cdsiriimia in the method of branching and from Bevocastriain the rectilinear course of the radial tubes and their much moreregular arrangement. The closest similarity is with ZonotrichitesBornemann from the Trias, but Zonotrichites apparently had thethreads packed more closely together and probably differedin the mode of branching. I am inclined to regard theresemblance as due to convergence.

RANGE. Lower Carboniferous to Trias (teste Pia 1937, p. 790).GENOTYPE. Mitcheldeania gregaria Nicholson.

:KEOGENOHOLOTYPE. Slide 7°741 in the Geological Surveycollection (ex-Wethered collection).

TYPE-LoCALITY. Kcrshopcfoot, Roxburghshire.

GARWOODIA GREGARIA (NICHOLSON).

(PI. 14, Figs. I and 2.)

Diameter of threads ranging between .05 and .072 mm.Distance between threads at points of closest packing rangingfrom 7 fL to 18 fL, approximately one-tenth to a quarter of thewidth of the tubes themselves. I These figures also give a measureof the length of the horizontal portion of the branches, sincethese branches turn upwards at a right angle to grow parallelto the other tubes as soon as the branch has become separatedby this amount from the parent thread. The typical nodulesof G. gregaria average about 10 ·12 mm. in diameter.

REMARKS. Ortonella kershopensis Garwood is constantlyassociated with material from the type locality, and as pointedout by Garwood (1931) represents the fine interstitial tubesfigured by Nicholson. The" mural pores" of Nicholson areshown in the specimen figured on PI. 15, Fig. I, and are clearlyplaces where the plane of the section cuts into the interior of atube at an undulation. The contrast between the clear calciteinfilling and the almost opaque fine grained calcite algal dustof the wall causes these portions to stand out as bright circlesalong the course of the tube.

I It is obviously impossible to pack cylinders so that they touch all round, hut the degreeof approximation of the tubes when two tubes appear to be cut longitudinally through theircentres probably has some significance.

LOW ER CARBONIFEROUS CALCAREOUS ALGAE. 223

T YPE-SPECDlEN A);V T YPE-LOCALITY. See description ofGaneoodia.

HORI ZON. Tucdian, top of Cementstone Group (e 2 ) . Alsokn own from the Whitehead Lim est one of the Forest of Dean (C 2 ) .

OTHER SPECIES OF GARWOOD/A.The generic descrip tion of Garwoodia is purposely drawn

rather closely, and it is impossible to say how man y of theprevious records of " Mitcheldeania sp." really belong toGarwoodia. In th ese lime-precipitating algae so few characte rsremain of diagnost ic va lue th at th e differential characters visiblemay be considered to have a high systematic value. Certainfeatures, such as size of th e tubes, may vary in a particulargenus, but others, such as th e mode of branching and even th eaggregation of the threads may be traces left by characters ofeven more than generic va lue. Within limits then, it is feltthat generic diagnoses should be narrowly drawn; only thuswill an idea of the past richness of these algal floras emerge.Concern ing the species previously figured as Mitcheldeania thefollowing remarks ma y be made.

Mitcheldeania? cribiformis Eth. fil. (I 909). Etheridge wasun doubtedly misled by Nicholson's description. This speciesdoes not belong to Ganeoodia and is probably a coelentera te,alt hough some porti ons of th e description read strangely likean account of a fossil alga.

Mitcheldean ia zonata Derville (193I ) probably belongs tothe genus Garwoodia. A suggestion of the cha racte risticbranching can be seen in Derville's figures. Th e " ampoules"described by Derville ma y be median sections of th e algal tubes,while the regions between, with narrower tubes and a greaterp roportion of dark interstitial material ma y be regions wherethe tubes lie slightly to one side of th e plane of the sect ion.The zonary structure on which th e specific name is based maytherefore be caused by a gentle undulation of th e radial threads.The regions of clear calcite (Derv ille's Fig. 39) I interpret as beingt he remains of soft organic matter which became attached tothe sur face of the nodule and was later overgrown.

M. capnostyloides Derville may be a Garwoodia which grewin relatively unfavourable conditions, but it is mor e likely to bean Ortonella or even an undescribed genus. The typ e figuresare not sufficiently good to enable this point to be decided .

Paul (? 1938) gave several figures of a Mitcheldeani a sp.from the middle Visean of th e Rhineland, but with pr aise­worthy caution refrained from giving it a specific name, sinceth e Brit ish species were so incompletely kn own. It is probable

224 A. WOOD,

that the specimen illustrated by figure 4 of his Plate I belongsto the genus Garwoodia. The threads in cross section are saidto average 31 p- wide and to range between 21 and 33 fL-thusshowing much the same wide range of variation as G. gregoria,but being markedly thinner. The characteristic branching maybe seen on portions of Paul's figure, and the course of thetubes as well as their close packing is closely similar to thatseen in the type species. The naming of this apparently newform must be left to those who have specimens available.

The figure of M itcheldeania sp. from the Km beds of theLower Avonmouth railway section given by Reynolds (1921,Plate X, Fig. I) may represent a new species. UnfortunatelyProfessor Whittard informs me that the original of this figurecannot be traced.

" M itcheldeania limestone" from Km, figured by the sameauthor (Plate VIII, Fig. I) contains Giroanella nicholsoni (Weth.)and another alga with radially arranged threads which maybelong to the genus Garwoodia. The specimens are small andapparently existed in somewhat unfavourable conditions.Such material cannot properly be used to found new species,and the investigation of Garwoodia in the K zone will haveto await the discovery of better preserved material.

SUMMARY.

M itcheldeania nicholsoni Wcthered, the type species of thisgenus, is found to be built up of a mixture of three genera of algae.One of these is chosen as typical and the name M itcheldeaniabecomes a synonym of Girvanella. "Alitcheldeania" gregariaNicholson, which is not represented in the nodules of 111.nicholsoni, becomes the type of a new genus Garwoodia and thespecies of this genus are discussed.

ACKNOWLEDGMENTS.

My thanks are due to Dr. C. ]. Stubblefield and Sir FranklinSibly for valuable help in the search for the type locality ofMitcheldeania, to Professor W. F. Whittard for the loan ofslides from the Reynolds and Wethered collections, BristolUniversity, and to Dr. T. S. Westall, Mr. A. G. Brighton andMr. R. B. Dent, who kindly searched for Mitcheldeania nicholsoniin the collections at Aberdeen University, the Sedgwick Museumand Gloucester Public Museums respectively. Mr. W. N.Edwards of the British Museum generously lent me preprintsof Paul's papers, the volumes containing them having not yetarrived in England.

LOWER CARBONIFEROUS CALCAREOUS ALGAE. 22$

REFERENCES.CHAPMAN, F. 1907. On the relationship of the genus Giruanella and its

occurrence in the Silurian limestones of Victoria. ReportAustralasian Assoc., Adv, Sci. 1907. (Separate copy only seen.)

DERVILLE, H. 1931. Les marbres du Calcaire Carbonifere en Bas­Boulonnais. Strasbourg.

ETHERIDGE, R., junr. 1909. An organism allied to M'itcheldeaniaWethered, of the Carboniferous Limestone, in the Upper Silurianof Malongulli. Ree. Geol, Suru. New South Wales, vol. viii, 308.

GARWOOD, E. J. 1913. On the important part played by calcareousalgae at certain geological horizons, with special reference to thePalaeozoic rocks. Ceol ..Mag., vol. I, pp. 440, 490, 545.

---- J9I4. Some new rock-building organisms from the LowerCarboniferous beds of Westmorland. Gcol, Mag., vol. Ii, p. 265.

---- 1931. The Tuedian beds of Northern Cumberland and Rox­burghshire east of the Liddel \Vater. Quart. [ourn, Geol, .ioc.,London, vol. lxxxvii, p. 97.

NICHOLSON, H. A. 1888. On certain anomalous organisms which areconcerned in the formation of some of the Palaeozoic limestones.Geol . 111ag., vol. xxv, p. 15.

PAUL, H. ? 1937. Unterkarbonische Kalka.lgcn und CalcisphaerenDeutschlands. J ahrb, Preuss. Geol, Landesanstalt Bd. 58, 1937,p.266.

? 1938. Algen und Spongiostromen aus dem rheinischen undenglischen Kohlenkalk. Decheniana tBerichte d. Niederrhein.geol. Vereins) Bd. 97A, p. 5.

PIA, J. VON. 1937. Die wichtigsten Kalkalgen des ]ungpalaozoikumsund ihre geologische Bedeutung. Compte Rcndus Congres Stratigr.Carbo Heerlen, 1935, vol. ii, p. 756.

REYNOLDS, S. H. 1921. The lithological succession of the Carboni­ferous Limestone (Avonian) ~f the Avon Section at Clifton.Quart. Journ. Geol, Soc. London, vol. lxxvii, p. 213.

SIBLY, T. F. and REYNOLDS, S. H. 1937. The Carboniferous Limestoneof the Mitcheldean area, Glouccstershirc. Quart. Journ. Geol .Soc. London, vol. xciii, p. 23.

WETHERED, E, 1886. On the structure and organisms of the LowerLimestone Shales, Carboniferous Limestone and Upper Lime­stones of the Forest of Dean. Geolc M ago vol. xxiii, p. 529.

---- 1887. Mitcheldeania Nicholsoni, A new genus, from theLower Carboniferous Shales of the Forest of Dean. PIOC.

Cotteswold Naturalists Club, vol. ix., p. 77.----. 1889. On the lower divisions of the Carboniferous Rocks of

the Forest of Dean. Proc, Geol, Assoc., London, vol. x, ,). 5"0.WOOD, A. 1941. "Algal dust" and the finer grained varieties of Car­

boniferous Limestone. Geol, Mag., vol. lxxviii, p. 192.

EXPLANATION OF PLATES 13-15.PLATE 13.

FIG. L Algal nodule showing the association of algae forming Mitchel­deania nicholsoni of Wethered. Closely comparable with thespecimen figured by Wethered in 1887 and considered by himto be a hydrozoan. Large and medium sized tubuli can beseen. X 20. Mitcheldean, Gloucestershire. Slide H.M.P·4482.

226 LOWER CARBONIFEROUS CALCAREOUS ALGAE.

FIG. 2. Girvanella [Mitcheldeania] nicholsoni (\Veth.) Neotype. Thelarge Cirvanella tubules ramify in a ground mass consisting oftwo other genera of algae. X 41. Brain's Qy. NO.2, nearDrybrook, Gloucestershire. Slide G.S.M. 70740.

PLATE 14.

;FIG. 1. Garwoodia gregaria (Nich.). Neotype and Genotype. The widevariation in diameter of the threads is well shown. Thesmallest tubuli are moulds of the threads of Ortonella kershop­ensis Garwood. X 25. Kershopefoot, Roxburghshire. SlideG.S.M. 70741.

FIG.2. Portion of the same enlarged to show the typical branching.X 38.

PLATE IS.

FIG. 1. Garwoodia gregoria (Nich.). Portion of the slide shown onPlate 2, with threads cut obliquely. The" mural pores" ofNicholson were probably described from a section cut in thismanner. X 43.

:FIG. 2. Mould of thread of Girvanella uicholsoni (Weth.) with precipitateof algal dust forming an apparent wall to the tube. In othercases the tube is buried in a thick opaque precipitate of algaldust. X 63. Slide W!4, Wethered collection, Bristol University.

FIG. 3. Branching in Gir oanella [M] nicholsoni (Weth.), showing thewide angle of divergence and possible polychotomy. X 35.Old Qy, Howle Hill, Glos. Sibly colI., G.S.M. Ze 2769a.

FIG. 4. Delicate threads of Ortonella twining round a Giruanella tubule inone of the cornposite Mitcheldeania nodules. X 37. OldQy., Howle hill, Glos., Sibly coll., G.S.M. Ze 276gb.

PROC, GEOL, Assoc. VOL. LII (1941), PLATE 13.

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2

Girnanclla lMitdzeldeania] n icholsoui (Woth.)

[To face p. 226.

PIWC. GEOL. Assoc., VOL. LII (1941). PLATE 14.

2

Caruroodia gen. nov.

LTo face p. 226.

PROC. GEOL. Assoc., VOL. LII (I94I). PLATE I5.

2

34

Ganooodia gen. nov., and Giruanella nicholsoni (Weth.)

[To face p. 226.