66 GEOLOGICAL SURVEY OF TEXAS. 



DIPLARTHRA. 



The only species of this order which have been discovered in the 

 Blanco beds belong to the genera Equus, Platygonus and Pliauchenia. 

 No trace of rhinoceros or tapir has been observed. 



EQUUS, Linn. 

 EQUUS SIMPLIOIDENS, Cope. 



Plate XX, Figures 1-3. 



Proceedings Amer. Philos. Society, 1892, p. 124. Annual Report 

 Geol. Survey of Texas, 1891 (1892), p. 252. 



Inferior molars and bones of this species are abundant at Mount 

 Blanco, but superior molars are less common, in a well preserved con- 

 dition. The exploration of 1891 yielded only two superior molars which 

 are determinable, and that of 1892 has yielded three which are determin- 

 able, though fractured, but none in better condition than the type. I shall 

 therefore confine my remarks on the present occasion to the inferior 

 molars. 



In a former paper I have given a synopsis of the distinctive characters 

 of the superior molars of the species of the genus Equus*. I now men- 

 tion characters which distinguish the inferior molars of some of the 

 species of the true horses which are identified by their undoubted asso- 

 ciation with superior molars. 



I. Metaconid and metastylid separated by a sharp entrant- 

 angular groove E. simpUcidens. 



II. Metaconid and metastylid separated by an open gutter 



as wide as the former at the middle of its depth E. occidentalis, 



E. argentinus (teste Burmeister) E. montanus, E. caballus. 



III. Metaconid and metastylid separated by a flat channel 



much wider than each of the former E. eurystylus, E. minutus 



The above character distinguishes the inferior molars of the E. sim- 

 pUcidens constantly, as I observe it in the entire series of two animals, 

 and numerous isolated teeth. From the South American Hippidiums, as 

 represented by Burmeister, they differ as they do from the species of 

 three-toed horses of the Loup Fork. The latter have a comparatively 

 small anteroposterior extent of metastylid and metaconid, and only a 

 slight concavity between them on the inner side of the crown. No 

 bones or teeth of the three-toed horse were found in the Blanco terranes. 

 Well preserved incisors show that this species is a true Equus, and not a 

 Tomolabis. 



♦proceedings American Philos. Soc, 1884, May. 



