1892.] NATURAL SCIENCES OF PHILADELPHIA, 435 



tofore described, though in position they correspond to the eres- 

 centic ridges which in Pi run from the deuterocone to the anterior 

 and posterior edges of the protocone. The posterior crest is best 

 developed and on Ps is especially prominent. In the lower series 

 pi is enlarged, simple and caniniform ; p^* and ^ are smaller and have 

 compressed, pyramidal protoconids, from the apex of which an 

 oblique ridge descends internally, ending in an exceedingly minute 

 deuteroconid ; a small talon is present on jA The last premolar has 

 the deuteroconid much increased and almost equalling the proto- 

 conid in height ; the low talon consists of the metaconid and a very 

 small tetartoconid. 



The genera Merychyus and Merycochcerus closely resemble Oreo- 

 don in the character of their dentition, but have made some 

 advances in the structure of the premolars. Thus in p^ and ° the 

 internal crescentic ridges have almost united and so, when some- 

 what worn, present an appearance not unlike that of Pj. On p^ the 

 deuteroconid is more distinct and on ^ it is separated from the pro- 

 toconid by a wider interval, producing a pattern quite like that 

 seen in the lower premolars of early members of the true ruminant 

 line. 



Leptauchenia exhibits again a different order of events. Pi is 

 very small and simple ; on P" and ■' the anterior crescentic ridge has 

 disappeared, while the posterior one is somewhat extended and thus 

 the crown of the tooth is invaded by a deep narrow valley which 

 runs backward and outward from the antero-internal angle. The 

 inferior premolars are simplified and reduced in size ; only p* dis- 

 plays a distinct deuteroconid and heel. Cyclopidius and Pithecistes 

 exaggerate the peculiarities of skull structure shown in Leptauche- 

 nia and it may be presumed that the same is true of their dental 

 characters, but the valleys are obliterated so early in these teeth 

 that I can state nothing further than the very obvious fact of a 

 great reduction in size of the premolars. 



In the tylopodan series the premolars of Leptoiragulus are not suf- 

 ficiently well known to detain us, and we may, therefore, commence 

 our survey with the White River genus, Poebrotherium. In this form 

 Pi has a simple and trenchant crown, which is remarkable for its fore 

 and aft extension. F^ and 3 are of the same general character, but are 

 even more elongated (antero-posteriorly) and Ps sometimes exhibits 

 the posterior internal crescentic ridge, though this is variable. P4 

 is of the ordinary selenodont pattern, composed of crescentic proto- 



