﻿198 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



Order PERISSODACTYLA 

 Family EQUIDAE 



Genus HYRACOTHERIUIM Owen 



HYRACOTHERIUM VENTICOLUM Cope 



Figure 78 



U.S.N.M. No. 18368, left mandibular fragment with Dp4 (fig. 78) 

 (loc. No. 48FE65) ; U.S.N.M. No. 18462, right mandibular fragment 

 with the heel of Dpg and Dp, (loc. No. 48FR76) ; U.S.N.M. No. 

 18461, right mandibular fragment with P2-4 and M3 (loc. No. 

 48FR78). 



Although this material is very fragmentary, it shows the character 

 of the lower deciduous premolars, which was not treated in Granger's 

 (1908) revision. Unfortunately, only the heel of Dp., is preserved. 



USNM -ia36& 



Figure 78. — Hyracotherium venticolum Cope, U.S.N.M. No. 18368; lateral and occlusal 



views of right DP4, Xl>i 



The characters that this limited material presents are as follows: 

 Dpa with posteroexternal crescent well developed, cross-crest between 

 hypoconid and entoconid well developed, hypoconulid small but dis- 

 tinct, external and posterior cingula present; Dp, with anterior and 

 posterior external crescents well defined, metaconid distinctly twinned 

 and higher than protoconid, cross-crest between hypoconid and 

 entoconid well developed, entoconid higher than hypoconid, hypoconu- 

 lid small but well defined, well developed anterior, external, and 

 posterior cingula. 



The deciduous teeth described here are somewhat higher crowned 

 than the permanent teeth, and can be distinguished from the perma- 

 nent dentition of Oroh/'ppvs onh^ with difficulty. In fact, these 

 teeth were originally referred to that genus and it was only after Dr. 

 C. L. Gazin and I spent some time comparing them with the material 

 in the U. S. National Museum that their true identity was learned. 



I have been told by Morris Skinner that this characteristic — the 

 deciduous teeth of horses being more advanced than the permanent 

 teeth — has been observed in the later Tertiary horses. In view of the 

 growth processes involved in the formation of horse teeth (White, 

 1942, p. 26) it is logical to correlate the above phenomenon with the 

 activities of the endocrine glands, which stimulate and regulate 



