212 Geological Sociely, 



not anchylosed to the substance of the jaw ; from the Ichthyosaurua 

 and all the lacertine Sauria in being implanted in di^thict sockets, and 

 not in a continuous groove ; from the Plesiosaurus and crocodilean 

 reptiles from the fangs not being simple and expanding as they de- 

 scend, 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 and 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 re- 

 garded as affording an exceptional example among Reptilia 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 com- 

 paring it with that of the teeth of other animals. 



In the Sphyrsena and allied fossil fishes which are implanted in 

 sockets, the teeth are characterized 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 higher 

 reptiles, the crown is also covered with enamel and the base with 

 csementum. But in the Cachalot and Dugong the whole of the 

 teeth is covered with csementum. In the Dugong this external layer 

 presents the same characteristic radiated purkingian corpuscles 

 or cells as in the csementum of the human teeth, and those of other 

 animals ; but the csementum of the Dugong differs from that of the 

 Pachyderms and Ruminants in being traversed by numerous calcige- 

 rous tube^, the corpuscles or cells being scattered in the interstices 



