302 SCOTT. [Vol. V. 



that the genus Equns has had a dual origin, on the one hand 

 from Hipparion, and on the other from Protohippus, involves a 

 degree of convergence which cannot be admitted in the present 

 state of our knowledge. It follows, therefore, that results drawn 

 from such a doubtful phylum can be of little service, and on that 

 ground these genera of uncertain position will not be taken into 

 consideration. 



I have chosen Mesohippus as the subject of the following de- 

 scription for a twofold reason : in the first place, because its an- 

 cestral relation to the modern genus is extremely probable ; and 

 secondly, because its structure is the least fully known of almost 

 any of the horse series, and we may thus avoid the repetition of 

 well understood facts. The description is founded chiefly upon 

 a series of specimens obtained by the Princeton expeditions 

 of '82 and '90. I am also indebted to the kindness of Profes- 

 sor Cope for the loan of some of his beautiful material from 

 Colorado. 



The typical American species, M. Bairdi, was first described 

 by Leidy as belonging to Palceotherium, (No. 32, p. 122), but 

 shortly afterwards referred by him to Anchitherium, under which 

 name it is described in all of Leidy's subsequent publications on 

 the subject. In 1875, Marsh erected it into the type of a new- 

 genus, Mesohippus, with the following definition: "This genus 

 presents characters intermediate between Orohippus Marsh, and 

 Anchitherium von Meyer. The skull and teeth are very similar 

 to those of the latter genus and the dental formula is the same. 

 In the feet, however, the lateral digits are larger, the fifth meta- 

 carpal is represented by an elongated splint bone ; and the second 

 and third cuneiform bones of the pes are not co-ossified. The 

 type of the genus is MesoJiippus Bairdi — Anchitherium Bairdi, 

 Leidy." (No. 34, p. 248.) 



With regard to this description it may be noted that the fifth 

 metacarpal is present in Anchitherium, as is demonstrated by 

 the facet for it upon the unciform, but nothing is known as to 

 its shape. The other distinctions are obviously not of generic, 

 but only of specific value, and consequently the new genus has 

 not been generally accepted as such. I have shown elsewhere, 

 however, that the characters of the incisor teeth are sufficient 

 to generically separate the European form from that of the 

 American White River beds. (No. 49.) In the former they 



