304 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



may be learned that the dentine of the crown is not continued as a root, 

 but terminates at a point which is in a line with the alveolur border, 

 and does not enter the alveolus. Thus there is no "cement-clad" root, 

 although the peduncle of the tooth is composed of a variety of bono 

 approaching cementum. Leidy remarks, — "The fang ... is mainly com- 

 posed of vertical osseous fibres, pervaded by numerous vascular canals 

 pursuing the same course as the former. It is of much finer texture 

 than the bone of the jaw with which it is coossified," etc. The large 

 vascular canals of this structure place it on the boundary between ce- 

 ment and bone, and its external appearance justifies the denomination 

 bone which Leidy applies to it. 



Cuvier states* that in the Maestricht Mosasaurus the teeth in age 

 " become filled throughout their length, and are most frequently found 

 entirely solid. They complete their development in becoming attached 

 to the jaw by means of an osseous body, very different in structure from 

 that of the tooth, with which it is nevertheless intimately associated. 

 The successional tooth originates in a special alveolus produced at the 

 same time, and it penetrates the osseous body of the tooth in use. In 

 enlarging, the successional tooth finally detaches the osseous body from 

 the jaw with which it was organically united j the body by a sort of 

 necrosis being shed and carrying with it the tooth it supported. Grad- 

 ually the successional tooth, with its body, improperly' called its osseous 

 root, assumes the position from which the old one was removed." 



Subsequently Cuvier,t after remarking that "he had formerly com- 

 mitted the error of calling the osseous structure, connecting the tooth 

 with the jaw, the root," observes that " he had since recognized it to be 

 the dental pulp, which, instead of remaining soft as in mammals, be- 

 comes ossified and identified with the alveolus." Cuvier continues: — 

 " The tooth has no true root, but adheres strongly to the pulp which 

 secreted it, and is further held in connection with it by the remains of 

 the capsule which furnished the enamel, and which, by becoming ossi- 

 fied also, and uniting itself with the maxillary bone and the ossified 

 dental pulp, inserts and fixes the tooth with additional force." 



All this is well known to Professor Owen (see his Odontography); 

 hence I conceive this position to be simply one of erroneous interpreta- 

 tion. Analogically, the teeth of these reptiles doubtless jjossess a root, 

 but this part is not homologous with the roots of the teeth of other 

 vertebrata; hence my statement must be accepted, that the teeth of 

 the Pythonomorpha " possess no true roots". 



III. — THE AFFINITIES OF THE PYTHONOMORPHA. 



The summary of the relationships of this order with which I close my 

 account of it in the second volume of the Report of the United States 



* Ossemens Fossiles, ed. 4, t. 10, p. 134. 

 t Ossemens Fossiles, 136. 



