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WHITE RIVER SELENODONTS 



than that of Leptomeryx, and the angle approximates much more to the great 

 hook-like shape seen in Poebrotherium, 



The ulna and radius are coossified, but the manus, so far as it is known, 

 does not differ in any important way from that of Leptomeryx. There is no 

 cannon-bone in the pes, the two functional metatarsals (iii. and iv.) remaining 

 separate; vestigial splints representing mt. ii. and v. were probably present also, 

 but they have not yet been recovered. 



That Hypertragnlus is nearly connected with Leptomeryx, and at the same 

 time is a member of the Tylopoda, appears from its entire structure and from 

 a comparison with the typical members of that group which preceded or 

 accompanied it in time. It must be remembered, in considering the system- 

 atic position of these genera, that the early tylopodans differed in many 

 important respects from the modern representatives of the suborder. 



Hypisodus Cope. 



The little animals comprising this genus are the smallest known members 

 of the family. They differ from all the others in having hypsodont molars 

 and in the peculiar character of the anterior lower teeth, the canine and first 

 premolar having taken on the form and function of incisors, which thus appear 

 to be ten in number in the mandibular series. Very little is known of the 

 skeleton, but that little leads us to infer that the foot-structure was very simi- 

 lar indeed to the condition found in Leptomeryx, except that there was no 

 cannon-bone in the pes. 



Protoceras Marsh. 



Professor Marsh ('91, p. 82) has proposed the formation of a separate 

 family for the reception of this extraordinary genus, but this seems hardly 

 necessary, for in essentials Protoceras does not differ more from Leptomeryx 

 or Hypertragidus than they do from each other, despite its bizarre appearance. 



The dental formula is : 1^, Cy, Pf, Mf . The upper incisors have entirely 

 disappeared, but the canine is, in the males, a large curved tusk of D-shaped 

 section and abraded upon the posterior face ; the lower canine has gone over 

 to the incisors, and its place is taken, in the males, by p T . P 1 is isolated 

 by diastemata in front of and behind it; the other premolars, both upper and 

 lower, are much as in Leptomeryx, but their great elongation antero-posteri- 

 orly gives them a strong resemblance to those of Poebrotherium. The 

 molars also resemble those of Leptomeryx, and in the upper jaw they are 



