344 GEOLOGICAL SUEVEY OF THE TEERITOEIES. 



not yet been indicated, but is well displayed in a specimen of the jaw of 

 8. arapaJiovius, Cope. It is known from Harlan's description that a large 

 foramen issues on the inner wall of the jaw, opposite each root. The 

 fractured ends of the specimen exhibit the course of the canal which is- 

 sues at this forame a. It turns abruptly downward between the inner 

 wall of the jaw and the fang of the functional tooth, and not far from 

 the foramen, its course is interrupted by the crown of the successional 

 tooth. This is situated obliquely as regards the long axis of the jaw. 



It is thus plain that the successional appearance of teeth is different 

 in this genus from what I have described in the two genera preceding. 

 In them the foramen is wanting, and the young crown rises within the 

 pulp cavity of the functional teeth, as in the Crocodilia. In this genus, 

 on the other hand, it is developed outside of the pulp cavity and fang of 

 the old tooth, and takes its place, as in many Lacertilia and in the Fytlio- 

 n(ymorpka, by exciting the absorption of the letter. The conic form of 

 these fangs in Saurocephalus is appropriate to such a succession, and 

 their great length seems to preclude the nutrition of the young tooth 

 from their bases. The use of the foramina on the inner face of the jaw 

 is thus made apparent, viz : The nutrition of the successional teeth from 

 without. I cannot trace the canal below the crown of the young tooth 

 to the base of the pulp cavity of the old tooth, and there are canals in 

 the jaw below the latter, one of which probably carried the dental artery. 



Species of this genus are less abundant in the part of Kansas exam- 

 ined by me than those of the preceding genera. Two only have been 

 observed up to the present time, ^. arapahovius and 8. phlebotomus^ 

 Cope. 



PACHYRHIZODONTID^. 



This family of physostomous fishes differs from the last in the nature 

 of its dentition. Instead of elongate, conic fangs sunk in deep alveoli, 

 it has shorter and stout fangs occupying alveoli of which the inner side 

 and part of the anterior and posterior walls are incomplete. The teeth 

 are, in fact, more or less pleurodont, but the extremity of the root is 

 received into the conic fundus of the alveolus. 



The premaxillary bones are well developed, but the maxillaries are 

 more so, and enter largely into the composition of the border of the 

 mouth. There is a well-developed angle of the mandible, but no coro- 

 noid bone is preserved in the si^ecimeas. The coronoid region is, how- 

 ever, broken in all of our specimens. The other characters of the family 

 are not determinable from our imperfect materials. 



PACHYEHIZODUS, Agassiz. 

 Dixon's Geology of Sussex, 1850, p. 374. 



This genus was established by Professor Agassiz on a jaw-fragment 

 from Sussex, England, with a brief description. The Kansas remains 

 resemble this fragment in their corresponding parts, and I refer them 

 to the same genus for the present. 



The genus as seen in our fossils is defined as follows : Muzzle flat ; 

 premaxillary bones rather long,, with two larger teeth together near the 

 anterior end behind the usual external series ; maxillary and mandibles 

 with a single series of simply cylindric, curved teeth 5 mandibular rami 

 closely articulated by a ligament. 



The teeth in this genus bear a superficial resemblance to those of a 

 mosasauroid genus. Their mode of succession appears to be as follows : 

 The crown of the young tooth was developed in a capsule at the base of 



