FOSSIL JAWS FROM STONESFIELD. 211 



considerably. In the manatee, the molars have two long and separate 

 fangs lodged in deep sockets, and the anterior teeth, when worn down, pre- 

 sent a form of the crown similar to that of the Basilosaurus, hut the oppo- 

 site 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 modifica- 

 tion 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 agree- 

 ment in the Basilosaurus with the known dental peculiarities of that class. 

 From the Mosasaurus the teeth of the American fossil differ in being im- 

 planted freely in distinct sockets and not anchylosed to the substance of 

 the jaw ; from the Ichthyosaurus and all the lacertine Sauria in being im- 

 planted in distinct sockets, and not in a continuous groove ; from the Ple^ 

 siosaurus and crocodilian reptiles from the fangs not being simple and ex- 

 panding 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 con- 

 tained within the cavity of the base of the protruded teeth ; but the Basi- 

 losaurus 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 Sphyrcena 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 cen- 

 tral, 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 ena- 

 mel, 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 co- 

 nical 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 ccsmentum. In the dugong this external layer presents 

 the same characteristic radiated purkingian corpuscles or cells as in the cce- 



