254 BIOLOGICAL LECTURES. 



not known. It would be of great interest to learn to what 

 extent the peculiar specialization of the cervical vertebrae had 

 proceeded. The vertebrae of the trunk and tail are like those 

 of the White River genus on a small scale. The limbs are 

 similar to those of the latter, but more primitive ; they, and 

 especially the feet, are less elongate ; the ulna and radius are 

 separate, except in old individuals ; the fibula, though extremely 

 slender, is still uninterrupted. In the manus are four func- 

 tional digits, and in the pes two, with the lateral pair reduced 

 to long filiform splints. Protylopus carries the main line of 

 tylopodan descent one stage farther back than had previously 

 been known, and, what is of even wider interest, it approxi- 

 mates this main line very distinctly to the other selenodont 

 groups above described, and which we have already allotted to 

 the tylopodan suborder. 



Another genus recently named by Wortman Leptoreodon 

 is very similar to Protylopus, but with most interesting and sig- 

 nificant differences. It has diastemata in the dentition, large 

 upper canines, and caniniform first lower premolar, as in the 

 oreodonts and Protoceras, and also, though in rudimentary fash- 

 ion, in Leptomeryx ; the limbs are rather short and the feet 

 tetradactyl. This genus nearly, but not quite, represents the 

 meeting point of the main tylopodan phylum, the Leptomery- 

 cidae, and the oreodonts, and greatly diminishes the gaps which 

 in White River times separated the three families. I regard 

 Leptoreodon as the probable ancestor of Protoceras, and I do not 

 know of any objection to such an arrangement, which would 

 explain the oreodont characters of the descendant genus. 

 These oreodont characters are so distinct that they led the late 

 Professor Cope to the conclusion that Protoceras was merely 

 an aberrant oreodont. While probable enough, the descent of 

 Protoceras from Leptoreodon cannot yet be proven, for the gap 

 between the two genera in structure and in time is still too 

 wide. The former has been found only in the uppermost divi- 

 sion of the White River stage (Protoceras beds), and we have as 

 yet found no ancestors for it in the middle and lower divisions. 

 Until these missing ancestors have been recovered, the relation 

 of Leptoreodon and Protoceras must remain somewhat uncertain. 



