rium they progressively diminish, as in the Myrmecobius. As the outer sides of the grinders in the jaw of the Phascolotherium are imbedded in the matrix, we cannot be sure that there is not a smaller cuspidated ridge sloping down towards that side, as in the crowns of the teeth of the Myrmecobius. But, assuming that all the cusps of the teeth of the Phascolotherium are exhibited in the fossil, still the crowns of these teeth resemble those of the Thyla cynus more than they do those of any placental Insectivora or Phoca, if even the form of the jaw permitted a comparison of it with that of any of the seal tribe. Connecting then the close resemblance which the molars of the Phascolotherium bear to those of the Thylacynus with the similarities of the ascending ramus of the jaw, Mr. Owen is of opinion that the Stonesfield fossil was nearly allied to Thylacynus, and that its position in the marsupial series is between Thylacynus and Didelphys. With respect to the supposed compound structure of the jaw of the Phascolotherium, Mr. Owen is of opinion that of the two linear impressions which have been mistaken for harmonic or toothless sutures, one, a faint, shallow, linear impression, continued from between the antepenultimate and penultimate molars obliquely downwards and backwards, to the foramen of the dental artery, is due to the pressure of a small artery, and that the author possesses the jaw of a Didelphys Virginiana, which exhibits a similar groove in the same place.Moreover, this groove in the Phascolotherium does not occupy the same relative position as any of the contiguous margins of the opercular and dentary pieces of a reptile's jaw. The other impression in the jaw of the Phascolotherium is a deep groove, continued from the anterior extremity of the fractured base of the inflected angle, obliquely downwards to the broken surface of the anterior part of the jaw. Whether this line be due to a vascular impression, or an accidental fracture, is doubtful; but as the lower jaw of the wombat presents an impression in the precisely corresponding situation, and which is undoubtedly due to the presence of an artery, Mr. Owen conceives that this impression is also natural in the Phascolotherium, but equally unconnected with a compound structure of the jaw; for there is not any suture in the compound jaw of a reptile which occupies a corresponding situation.

“The most numerous, the most characteristic, and the best marked sutures in the compound jaws of a reptile, are those which define the limits of the coronoid, articular, angular, and surangular pieces, and which are chiefly conspicuous on the inner side of the posterior part of the jaw. Now the corresponding surface of the jaw of the Phascolotherium is entire, yet the smallest traces of sutures, or of any indication that the coronoid or articular processes were distinct pieces, cannot be detected; these processes are clearly and indisputably continuous, and confluent with the rest of the ramus of the jaw. So that where sutures ought to be visible, if the jaw of the Phascolotherium were composite, there are none; and the hypothetical sutures that are apparent, do not agree in position with any of the real sutures of an oviparous compound jaw.

"Lastly, with reference to the philosophy of pronouncing judgment on the saurian nature of the Stonesfield fossils, from the appearance of sutures, Mr. Owen offered one remark, the justness of which, he said, would be obvious alike to those who were, and to those who were not, conversant with comparative anatomy. The accumulative evidence of the true nature of the Stonesfield fossils, afforded by the shape of the condyle, coronoid process, angle of the jaw, different kinds of teeth, shape of their crowns, double fangs, implantation in sockets, the appearance, he repeated, presented by these important particulars cannot be due to accident; while those which favour the evidence of the compound structure of the jaw, may arise from accidental circumstances."

"A paper was afterwards read, entitled 'Observations on the structure and relations of the presumed marsupial remains from the Stonesfield oolite,' by William Ogilby, Esq., F.G.S.

"These observations are intended by the author to embody only the most prominent characters of the fossils, and those essential points of structure in which they are necessarily related to the class of mammifers or of reptiles respectively. For the sake of putting the several points clearly and impartially, he arranged his observations under the two following heads:

"1. The relations of agreement which subsist between the fossils in question and the corresponding bones of recent marsupials and Insectivora. "2. The characters in which the fossils differ from those families. Mr. Ogilby confined his remarks to Marsupialia and Insectivora, because it is to those families only of mammifers that the fossils have been considered by anatomists to belong; and to the interior surface of the jaw, as the exterior is not exhibited in any of the fossil specimens.

"1. In the general outline of the jaws, more especially in that of the Didelphys (Phascolotherium) Bucklandii, the author states there is a very close resemblance to the jaw in recent Insectivora and insectivorous marsupials; but he observes that with respect to the uniform curvature along the inferior margin, Cuvier has adduced the same structure as distinctive of the monitors, iguanas, and other true saurian reptiles; so that whatever support these modifications of structure may give to the question respecting the marsupial nature of the Stonesfield fossils, as compared with other groups of mammals, they do not affect the previous question of their mammiferous nature, as compared with reptiles and fishes. The fossil jaws, Mr. Ogilby says, agree with those of mammals, and differ from those of all recent reptiles, in not being prolonged backward behind the articulating condyle; a character in conjunction with the former relation, which would be, in the author's opinion, well nigh incontrovertible, if it were absolutely exclusive: but the extinct saurians, the Pterodactyles, Ichthyosauri, and Plesiosauri, cotemporaries of the Stonesfield fossils, differ from their recent congeners in this respect, and agree with mammals. Mr. Ogilby is of opinion that the condyle is round both in Did. Prevostii and Did. Bucklandii, and is therefore a very strong point in favour of the mammiferous nature of the jaws. The angular process, he says, is distinct in one specimen of Did. Prevostii, and, though broken off in the other, has left a well-defined impression; but that it agrees in position with the Insectivora, and not the Marsupialia, being situated in the plane passing through the coronoid process and the ramus of the jaw. In the Did. Bucklandii, he conceives, the process is entirely wanting; but that there is a slight longitudinal ridge, partially broken, which might be mistaken for it, though placed at a considerable distance up the jaw, or nearly on a level with the condyle, and not at the inferior angular rim of the jaw. He is therefore of opinion that the Did. Bucklandii cannot be properly associated with either the marsupial or insectivorous mammals. The composition of the teeth, he conceives, cannot be advanced successfully against the mammiferous nature of the fossils, because animal matter preponderates over mineral in the teeth of the great majority of the insectivorous Cheiroptera, as well as in those of the Myrmecobius, and other small marsupials. In the jaw of the Did. Prevostii Mr. Ogilby cannot perceive any appearance of a dentary canal, the fangs of the teeth, in his opinion, almost reaching the inferior margin of the jaw, and being implanted completely in the bone; but in the Did. Bucklandii he has observed, towards the anterior extremity of the jaw, a hollow space filled with foreign matter, and very like a dentary canal. The double fangs of the teeth of Did. Prevostii, and probably of Did. Bucklandii, he says, are strong points of agreement between the fossils and mammifers in general;

but that double roots necessarily indicate, not the mammiferous nature of the animal, but the compound form of the crowns of the teeth.

"2. With respect to the most prominent characters by which the Stonesfield fossils are distinguished from recent mammals of the insectivorous and marsupial families, Mr. Ogilby mentioned, first, the position of the condyle, which is placed, in the fossil jaws, in a line rather below the level of the crowns of the teeth; and he stated that the condyle not being elevated above the line in the Dasyurus Ursinus and Thylacynus Harrisii, is not a valid argument, because those marsupials are carnivorous. The second point urged by the author against the opinion that the fossils belonged to insectivorous or marsupial mammifers, is in the nature and arrangement of the teeth. The number of the molars, he conceives, is a secondary consideration; but he is convinced that they cannot be separated in the fossil jaws into true and false, as in Mammalia; the great length of the fangs, equal to at least three times the depth of the crowns, he conceives, is a strong objection to the fossils being placed in that class, as it is a character altogether peculiar and unexampled among mammals; the form of the teeth also, he stated, cannot be justly compared to that of any known species of marsupial or insectivorous mammifer, being, in the author's opinion, simply tricuspid, and without any appearance of interior lobes. As to the canines and incisors, Mr. Ogilby said, that the tooth in D. Bucklandii, which has been called a canine, is not larger than some of the presumed incisors, and that all of them are so widely separated as to occupy full five-twelfths of the entire dental line, whilst in the Dasyurus viverrinus, and other species of insectivorous marsupials, they occupy one-fifth part of the same space. Their being arranged fongitudinally in the same line with the molars, he conceives, is another objection, because, among all mammals, the incisors occupy the front of the jaw, and stand at right angles to the line of the molars. With respect to the supposed compound structure of the jaw, Mr. Ogilby offered no formal opinion, but contented himself with simply stating the appearances; he, nevertheless, objected to the grooves being considered the impression of blood vessels, though he admitted that the form of the jaws is altogether different from that of any known reptile or fish. "From a due consideration of the whole of the evidence, Mr. Ogilby stated, in conclusion, that the fossils present so many important and dis tinctive characters in common with mammals on the one hand, and coldblooded animals on the other, that he does not think naturalists are justified at present in pronouncing definitively to which class the fossils really belong.'

"A paper was afterwards read, entitled, "Observations on the Teeth of the Zeuglodon, Basilosaurus of Dr. Harlan," by Richard Owen, Esq., F.G.S., Hunterian Professor in the Royal College of Surgeons, London. During the recent discussions respecting the Stonesfield fossil jaws, one of the strongest arguments adduced and reiterated by M. de Blainville and others in support of their saurian nature, was founded on the presumed existence in America of a fossil reptile possessing teeth with double fangs, and called by Dr. Harlan, the Basilosaurus. To the validity of this argument, Mr. Owen refused to assent, until the teeth of the American fossil had been subjected to a re-examination with an especial view to their alleged mode of implantation in the jaw; and until they had been submitted to the test of the microscopic investigation of their intimate structure with reference to the true affinities of the animal to which they belonged. The recent arrival of Dr. Harlan in England with the fossils, and the permission which he has liberally granted Mr. Owen of having the necessary secVOL. III.-No. 29. N. s.

tions made, have enabled him to determine the mammiferous nature of the fossil.

"Among the parts of the Basilosaurus brought to England by Dr. Harlan, are two portions of bone belonging to the upper jaw; the larger of them contains three teeth; the other, the sockets of two teeth. In the larger specimen, the crowns of the teeth are more or less perfect, and they are compressed and conical, but with an obtuse apex. The longitudinal diameter of the middle, and most perfect one, is three inches, the transverse diameter one inch two lines, and the height above the alveolar process two inches and a half. The crown is transversely contracted in the middle, giving its horizontal section an hour-glass form; and the opposite wide longitudinal grooves which produce this shape, becoming deeper as the crown approaches the socket, at length meet and divide the root of the tooth into two separate fangs. The two teeth in the fore part of the jaw are smaller than the hinder tooth, and the anterior one appears to be of a simpler structure.

"A worn-down tooth contained in another portion of jaw, Mr. Owen had sliced, and it presented the same hour-glass form, the crown being divided into two irregular, rounded lobes joined by a narrow isthmus or neck. The anterior lobe is placed obliquely, but the posterior parallel with the axis of the jaw. The isthmus increases in length as the tooth descends in the socket until the isthmus finally disappears, and the two portions of the tooth take on the character of separate fangs. It is evident that the pulp was originally simple, but that it soon divided into two parts, from which the growth of the ivory of the teeth proceeded as from two distinct centres, now separately surrounded by concentric stria of growth, the exterior sending an acute-angled process into the isthmus. The cavitas pulpi, which is very small in the crown of the tooth, contracts as the crown descends, and is almost obliterated near the extremity, proving that the teeth were developed from a temporary pulp.

"The sockets in the anterior fragment of the upper jaw are indistinct and filled with hard calcareous matter, but a transverse horizontal section of the alveolar margin proves that these sockets are single, and that the teeth lodged therein had single fangs. In the anterior socket, there is an indication of the transverse median contraction, showing that this tooth resembled in form, to a certain degree, the posterior tooth. A plaster cast of a portion of the lower jaw afforded the only means of studying this part of the fossil. It contains four teeth, of which the two posterior are nearly contiguous, the next is at an interval of an inch and a half, and the most anterior of two inches from the preceding. The last tooth is more simple in form than those behind, and it has been described as a canine. This fragment of the lower jaw thus confirms the evidence afforded by the fragments of the upper jaw, that the teeth in the Basilosaurus were of two kinds, the anterior being smaller and simpler in form, and further from each other than those behind.

"Mr. Owen then proceeded to compare the Basilosaurus with those animals which have their teeth lodged in distinct sockets; as the Sphyræna, and its congeners among fishes, the plesiosauroid and crocodílean Sauria, and the class Mammalia; but as there is no instance of either fish or reptile having teeth implanted by two fangs in a double socket, he commences his comparison of the Basilosaurus with those Mammalia which most nearly resemble the fossil in other respects. Among the zoophagous Cetacea the teeth are always similar as to form and structure, and are invariably implanted in the socket by a broad and simple basis, and they never have two fangs. Among the herbivorous Cetacea however, the structure, form, number and mode of implantation of the teeth differ

considerably. In the manatee, the molars have two long and separate fangs lodged in deep sockets, and the anterior teeth, when worn down, present a form of the crown similar to that of the Basilosaurus, but the opposite indentations are not so deep; and the entire grinding surface of the molars of the manatee differs considerably from those of the Basilosaurus, the anterior supporting two transverse conical ridges, and the posterior three. The dugong resembles more nearly the fossil in its molar teeth; the anterior ones being smaller and simpler than the posterior, and the complication of the latter being due to exactly the same kind of modification as in the Basilosaurus, viz. a transverse constriction of the crown. The posterior molar has its longitudinal diameter increased, and its transverse section approaches to the hour-glass figure, produced by opposite grooves. There is in this tooth also a tendency to the formation of a double fang, and the establishment of two centres of radiation for the calcigerous tubes of the ivory, but the double fang is probably never completed. The teeth in the dugong moreover are not scattered as in the Basilosaurus.

"Mr. Owen then briefly compared the teeth of the fossil with those of the Saurians, and stated that he had not found a single instance of agreement in the Basilosaurus with the known dental peculiarities of that class. From the Mosasaurus the teeth of the American fossil differ in being implanted freely in distinct sockets and not anchylosed to the substance of the jaw; from the Ichthyosaurus and all the lacertine Sauria in being implanted in distinct sockets, and not in a continuous groove; from the Plesiosaurus and crocodilian reptiles from the fangs not being simple and expanding as they descend, but double, diminishing in size as they sink in the socket, and becoming consolidated by the progressive deposition of dental substance from temporary pulp in progress of absorption. In the Enaliosauria aud the Crocodilia, moreover, there are invariably two or more germs of new teeth in different stages of formation close to or contained within the cavity of the base of the protruded teeth; but the Basilosaurus presents no trace of this characteristic saurian structure. From the external characters only of the teeth, Mr. Owen therefore infers, that the fossil was a mammifer of the cetaceous order, and intermediate to the herbivorous and piscivorous sections of that order, as it now stands in the Cuvierian system.

"In consequence however of the Basilosaurus having been regarded as affording an exceptional example among reptiles of teeth having two fangs, though contrary to all analogy, and as the other characters stated above, may be considered by the same anatomists to be only exceptions, Mr. Owen procured sections of the teeth for microscopic examination of their intimate structure and for comparing it with that of the teeth of other animals.

"In the Sphyraæna and allied fossil fishes which are implanted in sockets, the teeth are characterised by a continuation of medullary canals, arranged in a beautifully reticulated manner, extending through the entire substance of the tooth, and affording innumerable centres of radiation to extremely fine calcigerous tubes.

"In the Ichthyosaurus and crocodile the pulp cavity is simple and central, as in Mammalia, and the calcigerous tubuli radiate from this centre to every part of the circumference of the tooth, to which they are generally at right angles. The crown of the tooth in these saurians is covered with enamel, while that part of the tooth which is in the alveolus is surrounded with a thick layer of cortical substance. In the dolphins which have simple conical teeth like the reptiles, the crown is also covered with enamel and the base with camentum. But in the cachalot and dugong, the whole of the teeth is covered with camentum. In the dugong this external layer presents the same characteristic radiated purkingian corpuscles or cells as in the cœ