A. White River Selenodonts. 



Family LEPTOMERYCID/E. 

 Leptomeryx Leidy. 

 Plate I., Figures i, 2. 



IN this genus the dental formula is ; 1^, Of-, Pf , Mf . The number of upper 

 incisors is uncertain ; so far, only a single specimen has been found in 

 which part of the premaxillary is preserved, and in this there is the alveolus 

 for the lateral incisor, but whether the others were present also is not determin- 

 able. The upper canine is wanting, at least in the only two specimens which 

 would show this tooth if it were present, for in the immense majority of the 

 skulls which have been collected the delicate muzzle is broken across in front 

 of p- 2 -. It is possible, though it seems unlikely, that the absence of the upper 

 canine is a sexual character, and that the two specimens mentioned are of 

 females. On the other hand, no fragment of a tusk-like canine has ever been 

 found associated with the numerous individuals so far discovered. The number 

 of upper premolars is three, p- 1 having disappeared ; the others are quite com- 

 plex, each having a well-developed deuterocone, which is conical on p- 2 - and p 3 -, 

 crescentic on p*-. Viewed from the outer side, these premolars have a sharp 

 and trenchant form, while the prominence of the median external rib and of the 

 anterior and posterior buttresses gives them a trifid appearance. The upper 

 molars have incompletely formed internal crescents, and are remark-able for 

 the large size of the anterior and median external buttresses (or styles), and 

 especially for the great prominence of the rib upon the antero-external crescent, 

 which is of very unusual degree ; that on the postero-external crescent is far 

 less prominent. 



The lower incisors are procumbent and almost horizontal in position ; 

 the median incisor is remarkably long and straight, and is also the broadest 

 of the three ; i-j- is much shorter and more everted, and i^- is still shorter. 

 The lower canine has become an incisor in form and function and is consider- 

 ably shorter than i^, which it follows without a diastema. The first lower 

 premolar is isolated by a long diastema in front of and a shorter one behind 

 it ; it is very small and shaped like a canine, and its form immediately suggests 



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