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pointed tubercles upon the inner edge. Those specimens of this fossil which are completely disengaged from their matrix probably exhibit a similar conformation of their molar teeth; for M. Agassiz says in his note, that the grinders have five points, disposed like those of the Insectivora. The curve of the horizontal ramus of the Didelphis Bucklandii, and the form and direction of the ascending one, present a perfect agreement with that of the D. murina ; the differences consist in the fossil having the condyle less elevated, the tongueshaped process of the angle more external, and the opening of the dental canal more anterior.

The Didelphis Prevostic has the ramus of the jaw straighter, but the form of its molars, and the great number of these teeth, bring it nearer to the didelphs than to any other marsupial animal.

If we compare the fossil animal with the Insectivora, we perceive in the latter the coronoid process carried more forward, and separated from the condyle by a deeper space; the angular process of the jaw is shorter, forming a less obtise angle with the horizontal ramus; the commencement of the horizontal ramus is more convex, the rest of the bone straighter, and the number of teeth always less.

Nevertheless, if we admit that the fossil animal is of the order of Marsupialia, we must not wonder at the resemblance which

may exist between it and the Insectivora, for we know that the pouched animals form a kind of sub-class, as Cuvier says, of which the series is parallel with that of the placental Mammalia ; and we can thus distinguish insectivorous marsupials, carnivorous marsupials, and rodent marsupials; &c. But the animals of this order [Marsupialia) are the only the Cetacea excepted, which are furnished with so large a number of teeth.

It was also thought that this fossil animal might be referred to the family of the seals, on account of the subdivision of the teeth into lobules. I shall first observe, that in the Phoce properly so called, the common seal is the only one which has five tubercles upon the dental crown ;-that the others have only three ;—and that in the Phoca cristata there even appears to be nothing more than a simple, blunt, conical crown, furrowed upon its surface, and without any supplementary tubercles.

Thus, a lobulated form of tooth cannot be looked upon as a constant characteristic of the seals, and consequently is not a distinction of importance. But it must be observed that among the Amphibia the angle of the jaw is not produced into the tongue-shaped process which exists among the car

nassiers and the carnivorcus marsupials. In the common seal we find a simple tubercle at the niaxillary angle; in the Phoca cristata this process is more obtuse; and in the Phoca leptonyr, de Blainville, it is quite obsolete.

We see indeed that this process re-appears and becomes a character of more importance in the genus Otaria, in which it constitutes a strong, trihedral projection, obtuse, and prolonged into a prominent ridge below the jaw. But there is one characteristic mark in the species of this genus, which quite removes all affinity to the fossil jaw ;-their molar teeth bave but a single root.

Thus the supposed Didelphis does not appear to be referable to the family of the seals.

As we never see this angular process disappear in the carnassiers, I think we may therefore conclude that the fossil bones found at Stonesfield belong to a terrestrial carnivorous mammal; and on account of the great number of its teeth, that it is more closely related to the didelphs than to any other known mammiferous animal.

The present investigation furnishes a fresh proof that the attentive study of even the smallest parts of organic structure leads to very curious general results, since they become characters, the importance of which we did not in the least anticipate.

The prolonged tongue-shaped process is absent in man, in the Quadrumana, and in the frugivorous bats, animals in which the articulation of the jaw does not require that fixedness which is a necessary condition in the existence of the carnassiers. This process in the last furnishes a strong insertion for the ligaments or sets of muscles which regulate the lateral movements of the jaw; when it closes, they fix it in its articulation, and produce that action of the teeth necessary for the proper mastication of the food. This process is obsolete, or nearly so, in those seals which are placed in the order carnassiers, because these seize their prey in the water, and transfix it with their pointed teeth rather than masticate it, and do not therefore require so much fixedness of articulation.

If we observe it to become projecting among the Otarie, it is easy to account for this by a simple examination of their slightly pointed teeth, inserted obliquely and across the dental arch, and which would have been less fitted for retaining living prey, if the lower jaw had been capable of making a lateral movement below the upper one.

Were I not afraid of wandering from my subject, it would be easy for me to demonstrate that the prolongation of the

angle of the jaw is just as well adapted in the Rodentia for the action of their teeth.

Thus the form of this process, and that of the teeth and of the condyle, are always combined in such a manner that the study of these parts becomes of very great importance in ascertaining the natural relations of animals.

I think, therefore, to return to our subject, that the bones from the Stonesfield slate, published under the names of Didelphis Prerostii and Did. Bucklandii, have belonged to mammiferous animals, very nearly approaching the didelphs, but of a distinct genus.

Not having had the advantage of inspecting the portion of a jaw preserved at “l'Ecole des Mines,” I have been unable to treat of that fossil in this memoir.

M. Agassiz, who regarded these animals as of an ambiguous nature among Vertebrata, bas proposed for a generic name that of Amphigonus.

M. de Blainville, adopting the same views, without being aware of the name proposed by M. Agassiz, which is not cited in his note in Bronn and Leonhard's Journal, has proposed that of Amphitherium or Heterotherium. As in all that we can deduce from a study of the portions of jaws submitted to our examination, I see nothing which indicates an ambiguous or heterogeneous nature,--and as the names proposed by these naturalists express doubts which in my opinion no longer have any foundation, I think it would be advisable now to apply a more significant appellation. I do not think that sufficient time has elapsed for the ill consequences to arise which generally follow changes of names in Natural History, because those which I propose to replace by others have not yet been adopted by systematic writers, and consequently have not yet received the sanction of naturalists in general. The name of Thylacotherium appears to me a preferable one.

If we call to mind the figure of the fossil jaw published by Mr. Broderip, which is taken from a fragment that I have not examined, the new genus of fossil Mammalia will have the following characters, taken from the examination of the lower jaw only.

Eight incisor teeth, two canines, and ten molars, with five or six false anterior ones; the hinder teeth presenting a summit consisting of five tubercles, three internal and two external, the latter succeeded by a small “talon.”

The two species referable to this genus, areThylacotherium Prevostii, (Didelphis Prevostii, Cuvier),

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having its horizontal ramus some what straightened; its depth about the fourth part of its length. And

Thylacotherium Bucklandii, (Didelphis Bucklandii, Broderip), having its horizontal ramus narrower and more curved.

Such are the zoological characters at present known of this genus of fossil Mammalia.

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Art. II. - Notice of U'ndescribed Zoophytes from the Yorkshire

Chalk. By Joux EDWARD LEE, Esq. Professor Phillips, in his “Illustrations of the Geology of the Yorkshire Coast," has observed, that “the interesting remains of Spongia are nowhere so well developed as in England, and perhaps nowhere in England, so well as in Yorkshire. On the shore near Bridlington, they lie exposed in the cliffs and scars, and being seldom enclosed in flint, allow their organization to be studied with the greatest advantage."

This locality however, does not seem to have attracted the attention it deserves : the chalk cliffs from Sewerby to the Danes' Dyke on the south of Flamborough Head abound in Zoophytes, and a diligent collector will not be long in obtaining an extensive suite of specimens ; the chalk is of such a nature as to admit of being easily worked, so that the fossils may be cleared without much difficulty, and their characters properly exposed. The labour however has only commenced : the varieties in form, and the gradations from one to another, are almost endless, and the difficulty in determining species is so great, that it almost operates as a bar to the study of these remains; still as every additional fact respecting them must be of some value, where so little comparatively is known, I shall endeavour to give a description of several species which appear to me to be new; and should it afterwards prove that I have been mistaken, they can then be referred to their proper situations. Two of the species described seem to be Siphoniæ; four, or perhaps five, may for the present be considered as sponges, and one seems to be a Udotea.

It is a curious fact, that though the locality from which these fossils were obtained, is extremely rich in Zoophytes, yet the rest of the Yorkshire chalk is comparatively barren : this is particularly the case with the southern part of the range; I have sought almost in vain, for any specimens worthy of preservation, in the numerous chalk pits from Market Weighton to Hessle.

to any

The kind most abundant near Bridlington is the Spongia radiciformis of Phillips; numbers of this species lie in all directions in the cliff below Sewerby, both parallel with, and across the direction of the strata : many specimens appear to have been a good deal worn before they were imbedded, while others, particularly of the cup-shaped form, are perfect, even to the finest fibres of the root. In some cases these latter have disappeared, but are yet shown very beautifully by the hollows in the chalk they once filled, being coloured with ochreous matter. I have never yet observed the root of any sponge attached

of the other fossil bodies which are found in the chalk; this fact appears singular, since the fine fibres of the root are in many cases perfectly preserved: about two years ago however, I obtained a specimen of a variety of Špongia radiciformis, (or perhaps a new species), in which the short thick fibres of the root appear attached to the head of another individual of the same species.

The variety in outward form has been already referred to; the internal structure also exhibits very great irregularity of character : for instance, it has been generally believed that the root-shaped sponges had a central cylindrical cavity, extending downwards to the stem : the annexed diagram (fig. 1.)

shows however that this character is not constant: it is a magnified representation of the section of one of these fossil bodies. The specimen, when obtained, was broken in two or three places;

in the lower part of the stem there was a simple circular perforation, but about an inch higher this cavity had assumed a quadrangular appearance, and other circular canals were visible on each side, two of which were of much larger size than the others. At first sight, it appeared to connect the Siphoniæ with the Choanites of Mantell, and being very anxious to obtain further specimens, I examined with great care the neighbourhood of the spot where this fossil was procured: from that day to this however, I have never been able to find another instance of such a conformation, and at length I have come to the conclusion, that these characters must be considered as merely accidental.

The young Spongiæ are very abundant along the whole face of the cliff: a great variety of globular specimens may be obtained, from the size of a small pea to that of a common nut; the form then becomes rather conical, and there are often appearances of a process of attachment.-As they increase in size the specific characters gradually develope them

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