Geological Society. 805 



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 liaving 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 Sphyrana and allied fossil fishes which are implanted m 

 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 

 Sau'rians 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 

 c^mentum. But in the Cachalot and Dugong the whole of the 

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

 presents the same characteristic radiated purkingian corpuscles 

 or cells as in the c^mentum of the human teeth, and those of other 

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

 Pachyderms and Ruminants in being traversed by numerous calcige- 

 rous tubes, the corpuscles or cells being scattered in the interstices 

 of these tubes. Now the crowns of the teeth of the Basilosau- 

 rus evidently exhibit in many parts a thin investing layer of a 

 substance distinct from the body or ivory of the tooth, and the mi- 

 croscopic examination of a thin layer of this substance proves it to 

 possess the same characters as the caementum of the crown of the 

 tooth of the Dugong. The purkingian cells are, in some places, 

 scattered irregularly, but in others are arranged in parallel rows. 

 The tubes radiating from the cells are wider than usual at the com- 

 mencement ; but soon divide and subdivide, forming rich reticula- 

 tions in the interspaces, and communicating with the branches of the 

 parallel larger tubes. These are placed, as in the Dugong, perpen- 

 dicular to the surface of the tooth, but they are less regularly arranged 

 than the calcigerous tubes of the ivory, with which, however, they 

 form numerous continuations. There"is a greater proi)ortion of cae- 

 mentum in the isthmus of the tooth than elsewhere ; and the worn- 

 down crown of the tooth must therefore have exhibited a complicated 

 structure. The entire substance of the ivory of the teeth consists 

 of fine calcigerous tubes radiating from the centres of the two lobes, 

 without any intermixture of coarser medullary tubes which charac- 

 terize the teeth of the Iguanodon ; or the slightest trace of the re- 

 ticulated canals, which distinguish the texture of the teeth of the 

 Phil. Mag. S. 3. Vol. 14. No. 89. April 1839. X 



