TRANSACTIONS OF WAGNER 

 l8 



WHITE RIVER SELENODONTS 



available it is extremely difficult to work out the homologies of the incisors 

 and canines, but the arrangement appears to be as follows : The upper incisors, 

 if present at all, are quite unknown, for no specimen has yet been found with 

 uninjured premaxillaries. The upper canine was preserved, as is indicated by 

 the alveolus in one individual. The lower incisors are small and very delicate 

 and somewhat less procumbent than those of Leptomeryx. The lower canine 

 is not incisiform, and is separated from io^ by a very short diastema; it has a 

 high, slender, pointed, and recurved crown, which is distinctly larger in some 

 specimens than in others, a difference which is doubtless sexual. The pre- 

 molars are much simpler than in Leptomeryx. Y- is a simple, sharp, com- 

 pressed cone, implanted by two widely divergent fangs. P^^, which follows p- 

 after a considerable diastema, is similar but somewhat smaller ; p'^ is supported 

 upon three fangs and bears a small internal cusp (deuterocone) ; p^ is, like 

 that of Leptomeryx, composed of two crescents, internal and external. Unless 

 what seems to be the lower canine should prove to be a caniniform premolar, 

 which is extremely improbable, then py has disappeared ; p^ is simple, com- 

 pressed, and conical, implanted by two fangs, and isolated by a long diastema in 

 front of it and a much shorter one behind ; p^^ is high, acutely pointed, and 

 simple, but pj has anterior and posterior basal cusps and a small though 

 distinct deuteroconid. The premolars of Hypertragjilus, both upper and 

 lower, are distinguished from those of Leptomeryx not only by their greater 

 simplicity of structure, but also by their much smaller extension in the antero- 

 posterior direction. The molars are much like those of the last-named genus, 

 and the superior ones have the same extraordinarily prominent median rib 

 upon the antero-external crescent, but the outer buttresses are smaller and 

 project less. 



The skull (Plate I., figs. 3, 4) has quite as characteristically a tylopodan 

 appearance as that of Leptomeryx, though in a somewhat different fashion. 

 The cranium is well-rounded and capacious, with low and short sagittal crest, 

 and the forehead is broad ; the face narrows anteriorly more abruptly than in 

 Lepto7neryx, so as to give it a more llama-like appearance. On the other 

 hand, the muzzle is not nearly so long and tapering, which makes quite a dif- 

 ference in the general look of the two skulls. The orbit is left rather widely 

 open behind, because of the absence of a postorbital process from the jugal, 

 and the facial vacuity between the frontal, nasal, lachrymal, and maxillary is 

 larger than in Leptomeryx. The top-view of the skull is of strikingly tylopo- 

 dan character. (Plate I., fig. 4.) The mandible has a shorter horizontal ramus 



