68 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [VolY. 



the basis of our knowledge of tliis species. It presents the characters of 

 the genus in the following superior dental formula: I. 3; C. 1; P.m. 4; 

 M. 3. The first premolar is situated in the middle of a long diastema, 

 and a short one separates the canine from the third incisor. 



On comparison of this species with the P. heterodontus, the tyjie, and 

 heretofore the only known species of the genus, various characteristic 

 peculiarities may be observed, which will be noticed in the course of the 

 description. It is considerably smaller than the P. heterodontus, resem- 

 bling in its dimensions the 'Procamelus occidentalis. 



The crown of the secoipid superior incisor is directed forwards, and the 

 cutting edge is obhque to the long axis of the tooth. The first incisor is 

 equally large, and its alveolus occupies the apex of the premaxillary 

 bone. In P. heterodontus, the alveolus is smaller, and the apex extends 

 considerably beyond it. The third incisor has a conic crown, with sub- 

 round section. In P. heterodontus it is more robust, and is oval in sec- 

 tion, with weak posterior cutting edge. The canine is less robust than 

 the third incisor, and is about as far posterior to it as the latter is fi?om 

 the second incisor. The cro^oi is slightly compressed, and is less robust 

 than that of P. heterodontus. The first premolar is still weaker, and the 

 crown is compressed; the roots are only discrete at their extremities. 

 It is situated a httle more than one-third the distance between the canine 

 and second premolar behind the former. The second premolar is well 

 developed, and is two-rooted. The third premolar is also large, with 

 the grinding surface of the crown about half as wide as long. It has a 

 strong internal basal cingulum, which on attrition encloses a groove-hke 

 fossa with the principal crown. The external face of the crown is gently 

 convex between an anterior and a posterior ridge. The internal face of 

 the crown is uniformly convex. The fourth premolar has both crescents 

 well developed. Its grinding face is subsemicircular, and there are a 

 strong anterior and a weak posterior external vertical ridge. In P. hete- 

 rodontus, the grinding surface of this tooth is more nearly subquadrate. 



The true molars are subquadrate in horizontal section, and have short 

 crowns, well distinguished from the roots. The anterior horn of each ex- 

 ternal crescent is prolonged, constituting a section of a prominent verti- 

 cal external ridge of the crown at each point. The external sides of the 

 columns are but shghtly convex. The inner sides of the internal col- 

 umns are strongly convex. The enamel borders of the lakes are abso- 

 lutely simple, and there are no included enamel fossae. The posterior 

 outer angle of the last superior molar is not produced. 



As compared with the true molars of P. transmontanus, those of the 

 P. heterodontus are relatively smaller in transverse diameter. The mas- 

 ticating surfaces of the crowns of the second and third are thus more 

 elongate in outline. They are also rather more prismatic, and the last 

 two apparently occupied longer time in the process of protrusion. They 

 are much larger than those of P. transmontanus. 



The foramen infraorUtale issues above the middle of the fourth pre- 



