Adelaide Philosophical Society

PAPER READ SEPTEMBER 5TH, 1865, BY THE REV. J. E. TENISON WOODS, F.G.S. F.L.S., &c.

SUBJECT - "THE TERTIARY ROCKS OF SOUTH AUSTRALIA."

PART III. - BRACHIOPODA.

The Brachiopods are bivalve molluscs, having one shell placed on the back of the animal and the other in front. These shells are always unequal in size, but symmetrical, and the ventral or larger valve is distinguished by a perforation somewhere near its summit, through which a ligament is passed for the attachment of the animal. The valves are articulated by two curved teeth, so completely interlocked that they cannot be even opened to a moderate width without breaking the shell. As I am now engaged upon an account of fossils I shall not delay with any reference to the anatomy of the animals, which have been fully treated in the works here referred to. The best treatise on the Brachiopoda is to be found in Davidson's Monograph in the Palaeontographical Society's pub­lication for 1853. The introduction contains memoirs from Professors Owen, Carpenter, &c., and while giving the most recent observations is completely exhaustive of the subject. The best compendium is to he found in Woodward's Manual of the Mollusca. The latter is amply sufficient for all ordinary purposes of reference. It will be necessary to mention that the name Brachiopoda was proposed by Cuvier, in 1805 (Ann. du Mus., vol. I, p. 44), from the two variously-curved and cirrated arms or labial appendages with which each animal is provided. These arms not only create currents to bring them food; but also pass fresh supplies of oxygen over the mantle, which, in the absence of any special breathing apparatus, per­forms the office of respiration. Owing to this one anatomical peculiarity these shells are distin­guished from all others by the possession of an internal skeleton, on which the arms are sup­ported. The various ways in which this skeleton is modified in different species has formed lately the basis on which their classification is esta­blished; and as to this is due the structure of the animal, we possess more knowledge of the animal than of any other class of mol­luscs, and it follows that the classification is more natural and more strictly in accordance with sound zoological principles. Fortunately, too, the work has been taken in hand by the most eminent palaeontologists we have, and numbers amongst its labourers the names of Cuvier, Dumeril, Von Bach, D'Orbigny,. De Verneuil, Gray, McCoy, and Davidson. It is to the latter, however, we owe a complete restoration of the nomenclature and classification; for, unfortu­nately, the number of investigators increased the number of the synonyms most terribly, and it fre­quently happened not only that one specimen re­ceived three or four different names from different naturalists, but that even specimens of the same fossil would receive three or four different names from the same observer. To show, however, how enormous is the work in this matter, even though the investigations refer to only one class of a sub­kingdom, it may be mentioned that Mr. Davidson has been obliged to extend his observation to more than one series of papers in different publications, and not only are the splendid monographs of the Palaeontographical Society insufficient for the pur­pose, but even the numerous and lengthy papers in the Society's Journal, the Geologist and the Geological Magazine leave the work still incomplete.

The Brachiopoda are, according to the author just named, the most elegant in their shapes and the most abundantly distributed of any in the Molluscan sub-kingdom. They are found in the oldest deposits at present known to contain vestiges of animal life, and have continued to exist, some in similar and many under different shapes to the existing forms. Their value to the geologist is consequently very great, because they appear in strata which contain no other fossils, and they possess in each geological period marked peculiarities of form, which enable the palaeontologist to determine the age of any given deposit by their aid alone. All the various changes of form are more or less connected with the loop or internal skeleton for the brachial appen­dages, and it is perfectly astonishing how many varieties it has undergone. To describe the dif­ferent modes in which the typical skeleton is re­presented would be a very lengthy task; but its magnitude may be estimated from the fact that about 43 genera and sub-genera are known, which between them number perhaps 1,000 species. But not only in time do the Brachiopoda enjoy a great range, but also in climate. They are found in tropical and polar seas, in pools left by the re­tiring tide, and at the greatest depths hitherto explored by the dredge. (Woodward, p. 299, et seq.) At present only 70 species are known as actually living; but many more may yet be found in deep water, because these localities are necessarily but little explored as yet. and the Brachiopoda are found more in deep water than anywhere else. Seventy species is said to be a larger number than any known to live at one time in the secondary system: but the class seems to have obtained its maximum of development in the Devonian or third great geological epoch of the primary rocks, and consequently in the very early dawn of the earth's history. There appears clearly to have been a great development, and then, after a thinning out, a gradual increase of representative species up to the present state of the earth's sur­face - a fact which would seem to be less easily reconciled with the Darwinian theory than any other.

Another peculiarity of the Brachiopoda is that the shells are studded with small perforations, into which short tubes of the mantle are prolonged. This enables the microscopist to determine frag­ments of the shells when they occur, as they frequently do in the Tertiary formation under consideration, mixed up with dust of Foraminifera and portions of Bryozoa. Dr. Carpenter states that the shell of the Brachiopoda generally con­tains less animal matter than other bivalves, but that Discina and Lingula consist almost entirely of a horny animal substance. There is not in these shells, nor probably in any of the Brachiopoda, a distinction between the outer and inner layers of shell which occurs in other bivalves. The loop processes are always solid and destitute of perfora­tions. Most of the shells of the class found in the Mount Gambier formation are semi-transparent, and fragments are completely so, showing their structure well under the microscope.

Though the Brachiopoda have representatives in every formation, from the oldest to the most recent, yet the peculiarities of the modem forms are well marked and distinct. With the exception of one or two genera, they are all provided with an internal skeleton in the form of a loop. Some of the very ancient genera have representatives in even Australian seas; but as far as my knowledge extends, the Mount Gambier and Portland beds do not contain any. All the species I shall describe have a well-developed loop, and this peculiarity extends to the exclusion of all those genera, such as true Terebratulae, which have only a very small one. I now proceed to describe the species.

   Family - TEREBRATULIDAE. Lihroyd, 1696. - (Ichnographia Lithophylarii Britannici.)

Animal fixed to submarine bottoms by a mus­cular peduncle issuing from a perforation in the beak of the larger or ventral valve. This aperture is partly surrounded by a "deltidium" in one or two pieces; oval appendages entirely or partially sup­ported by calcified processes which commonly assume the shape of a loop, variable in form and dimension, but always fixed to the smaller or dorsal valve; shell structure always punctated. Seven genera and five sub-genera, all based on well-defined modifications in the form and position of the calcified supports of the arms.

   Genus - TEREBRATULA.

Shell oval, elongated, or transverse; extremely smooth plaited valves, more or less unequally convex; margin even or waved; hinge-line curved; beak short, terminated by a foramen variable in size and partly margined by a deltidium in one or two pieces; loop short, confined to the posterior portion of the shell, and not exceeding much more than a third of the length of the valve, simply attached to the hinge-plate, the two band-shaped lamella are soon united by a transverse lamella bent upwards in the middle. The cirrated arms are supported by the crura, and project considerably in front of the loop; no internal septum in the dorsal valve.

Obs. - Some palaeontologists (King, Gray, &c., in opposition to D'Orbigny, Davidson, and others, who maintain that the difference is generic) seem disposed to consider the species with short loops are the types of the genus Terebratula, and propose to place into separate sections and sub-sections those forms in which the calcified supports before being reflected or turned back (Waldheimia) extend to near the margin, as well as those in which the crura unite in the form of a band behind the mouth of the animal (Terebratulina); but these subdivisions, if of little value, are very convenient in the arrangement of the species, which may be even distinguished by external characters. Thus, in those forms with long loops the mesial septum may easily be traced on the external surface of the smaller or dorsal valve, while none such occurs in the short-looped species. (Davidson, loc. cit. The italics are my own.)

As yet no true species of Terebratula have beer found in the Mount Gambier or Portland deposits. and this is the more singular as they are not un­common in the Lower Miocene formation of Hamilton. One or two well-known species occur in the older Pliocene of Europe, but they do not exist in the present period. In fact, there seems to have been a gradual extinction of these forms in the Tertiary period until there has been only one species left living at present. It is not an Australian shell, and therefore we need not be surprised at the absence of these shells from our older Pliocene beds. When I say absence, of course I do not mean to say that there are none; but as none have been found yet, they must be rare, and will thus

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prove the gradual extinction of this peculiar Brachiopod.

   Sub-Genus - WALDHEIMIA. King. 1849. - (A Monograph of English Permian Fossils, 1849.)

Shell variable in shape, more or less circular, sub-quadrate, transverse, or elongated, with both valves convex, or with the dorsal concave; margin straight or waved; surface smooth or plaited; beak truncated and perforated by a circular fora­men of variable dimensions, partly completed by a deltidium in one or two pieces; loop long, in general exceeding two-thirds of the length of the valve, formed of slender, shelly, riband-shaped lamellae, simply attached by the crura to the hinge-plate, and more or less folded back on itself. The cirrated arms are partially supported by this ap­pendage, and united throughout by a membrane exactly as in the typical species of Terebratula. The valves articulate by means of teeth situated one on each side of the deltidium, supported by plates, and fitting into sockets in the dorsal valve, in the interior of which is a prominent cardinal process and hinge-plate, with four depressions, under which originates a central mesial septum extending more or less into the interior of the shell. - [Davidson, loc. cit., p. 64.]

Waldheimia grandis. New species. Plate II Fig. 1. - A, ventral valve; B, dorsal valve, and foramen of ventral valve; C, deltidium. Shell, smooth, very thick, elongated, convex; ventral valve, sub-carinated margin with two obscure plaits in the older specimens; beak short, obtuse, obliquely truncated, with a large circular cup-shaped thickened foramen. Loop two-thirds as long as the dorsal valve, lamellae slender, straight, reflection unknown, septum as long as loop, gradually tapering, crura thick, semicircular. Length 2.2 inches; breadth, 1.4 inches. Mount Gambier. Rather rare.

Obs. - This is a very variable shell, but one whose thickened shell preserves it pretty perfect. I am not certain that it is properly referred to the subgenus Waldheimia, because I have only been able to dissect two specimens. In both these in­stances the fossil was so completely converted into selenite that I could only obtain a knowledge of the structure of the loop by grinding away the dorsal valve in small portions at a time. I could not ascertain if there was any attachment to the septum; but had there been any I think I could hardly have failed to perceive it. The lamellae were very slender, nor was there towards their centre any, thickening, such as would occur at a point of attachment. Some specimens which I have seen are strongly marked with lines of growth, while in others these are scarcely per­ceptible.

It is a singular confirmation to the general rule of parallel organisms in our Tertiary beds that we should find large Terebratulidae in the older Pliocene formations. In the same geological horizon in Europe we find Terebratula grandis, which has many points of resemblance to the fossil under consideration. As far as exter­nal characters go, one shell might easily be mistaken for the other. They are both large smooth and massive shells with obtuse posterior margins and large foramina; but the shell of T. grandis is thin and brittle, while that of W. grandis is very thick, and the posterior beak is much shorter in the European than in the Aus­tralian fossil, and the foramen larger. I have seen fragments of large Brachiopoda from the Murray beds, but could not state positively that they were identical with the species under consideration, though they resembled it closely.

Waldheimia Crouchii, new species. Plate II. Fig. 2. - The only specimen of this fossil which I have seen was a broken dorsal valve. It had seven large plaits upon the anterior margin, but these were only peculiar to the conditions of the animal, for the earlier lines of growth showed no such sinuations. Mere folds of this nature would not be sufficient distinctions on which to erect a species, because nearly all the larger or older Brachiopoda in this formation seem to possess them. As, however, the valve seemed to me to be flatter and the septum longer and thicker in pro­portion, with very small crura for the attachment of the loop. I have named it provisionally after my esteemed friend and fellow-labourer in these deposits, G. Godwin Crouch, Esq. The specimen was found at Mount Gambier. Length, 2.3 inches; breadth, 2.1 inches.

Waldheimia imbricata, new species. Plate II, Fig. 3 - A, ventral valve; B, dorsal valve. Shell elongated, convex, angular in front, with a broad depressed keel on the ventral valve; margin waved; surface strongly marked with imbricated laminated lines of growth, which become sinuous and plaited as they recede from the posterior margin; beak sharply curved, with an oblique cordiform foramen; deltidium ribbed transversely; loop lamellar; curve unknown. Septum half the length of the shell. Length, 1 inch; breadth, 0.7 of an inch. Locality, Mount Gambier. Not common.

   Sub-genus, TEREBRATELLA. D'Orbigny, 1847. ("Consideration Zool. et Geol. sur les Brachiopodes, Comptes Rendus de l'Academie des Sciences, 1847." There is a dispute as to priority, according to Mr. Davidson, between our own Professor McCoy and D'Orbigny. Mr. D. gives it in favour of the latter; but as far as I can gather I do not think that Pro­fessor McCoy referred to the same genus.)

Shell elongated or transverse, variable in shape; both valves regularly and unequally convex, or in­terrupted by a longitudinal depression in the dorsal valve; beak truncated by an oblique foramen of a circular or oval form, and partly margined by a deltidium in two pieces, at times disunited above the umbo; beak ridges more or less defined, and in some cases having between them and the hinge line a flat or concave cardinal area; external surface smooth and variously punctated; loop, lamellar, reflected, but instead of being free during the whole of its course, receives two horizontal lamellas at about the middle of the shell, which serve as supports.

Hitherto we have been dealing with a genus which has the loop free and detached from the septum. We come now to another, the peculiari­ties of which are as above described. It is not so extensively represented in species, but those which we do find are the commonest fossils of the beds. They are found in almost every foot of the stone, both at Mount Gambier and at Portland, and from this circumstance become a very good test to determine the contemporaneity of any deposits which are similar in mineral characters.

Terebratella compta. Sowerby (Phy. Des.. New South Wales, by Count Strzelecki, p 297). Plate II, Fig. 4. -  A, dorsal valve, with deltidium; B, Ventral valve; C, side view of both valves; D, interior of dorsal valve, showing reflection of loop and horizontal attachment; E, side view of dorsal septum and loop; F. horizontal lamellae.

This species was described by Sowerby as follows:- "Shell, smooth, thin, trapeziform; lateral margins sub-incurved, anterior margin small obtuse; hinge area large, with a longitudinal depressed line at either side; ventral valve triangular, rounded slightly, truncated in front with a small median sinus; dorsal valve faintly keeled; ligumental foramen terminal small and round." To this description I may add that the dorsal valve is sub-cordiform and flat, and both valves distinctly marked with concentric lines of growth. Septum round and solid, lamellas of loop widening to the point of attachment, becoming again con­tracted at the reflection, and then extending into an almost complete circle with a slight projection towards the hinge.

In classifying this shell attention was drawn by Mr. Sowerby to the faint heel on the dorsal valve. This is the septum seen through the shell as a darkened translucent line. The existence of such a mark is a sufficient indication that the fossil does not belong in the true Terebratulae.

Terebratella Tenisoni. new species. Plate II., Fig. 5. - A, ventral valve; B. dorsal valve; C, side view of both valves; D, side view of dorsal valve, showing prolongation of the septum; E, front view of dorsal valve showing septum and muscular impressions. Fig. 6. - Same magnified.

Shell elongated, unequally convex; ventral valve, trapseziform, keeled deeply, and terminating in a notch at the the anterior margin; dorsal valve orbicular, and tapering to a point which fits into the notch on the upper valve; beak obtuse; deltidium striated, trigonal; foramen oblong. Septum thickened, curved, and produced so as to touch the ventral valve; attachments of the loop at the centre and nearer to the shell than to the edge of the septum; muscular impressions deep, hinge and crura sloping away from the septum, with a deep sinus in the centre, giving it the ap­pearance of a cervical vertebra. Locality, Mount Gambier and Portland. A smaller species, pro­bably only a variety, found in Hamilton. Size, variable, but adult specimens 0.7, breadth 0.4 of an inch.

Obs - This curious species has strong points of resemblance to the T. Evansii, found, I believe, in New Zealand, to which it is closely allied, but whose individual characters are very distinct. In both the septum is produced so as to touch the ventral valve. The anatomy of the muscular sys­tem must have been peculiar to cause such deep impressions, and I have no doubt shut when this specimen is more carefully examined it will be found to possess features connecting the sub-genus with other more remote members of the family.

I cannot close this paper without expressing my obligations to Mr. J. R. Y. Goldstein, of the Land and Survey Office, Portland, through whose un­tiring assistance I have been able to ascertain the structure of the loops of the two species just described. It would be difficult to convey the care and nicety required in the examination. Fossils with perfect loops are not easily found, and in dis­secting them the touch of a hair will shiver the lamellae to pieces. They are the most fragile fossils I have ever seen. It is to the pencil of the promising young geologist whose name I have just mentioned that I owe the drawing of T. compta, and I believe he had to dissect 20 or 30 specimens before he succeeded in getting a knowledge of the structure. The loop of T. Tenisoni has not yet been seen.

General Observations. - The characters of the fossils I above described are decidedly recent, and though none of the species appear to exist at the present time yet they are intimately connected with them. Thus the development of the mesial septum in Terebratella Tenisoni corresponds with the same peculiarity in T. crenulata as well as T. Evansii, but in no other member of the genus is there anything exceptional about the septum. Waldheimia grandis is already indi­cated somewhat similar to Terebratula grandis of the European Pliocene, but the fossil has been found in the Upper Miocene as well. Nothing can be concluded as to the climate from these fossils, as they lived in a very deep sea. Here, as in Europe, there is evidence of a tropical character in the fauna of our earlier Miocene formations; but whether or not this indicates similar conditions of climate is certainly open to question.