THE MAMMALIA OP THE DEEP KIVER BEDS. 125 



ARTIODACTYLA. 



OreodontMae. 



MESOREODON Scott. 



Amir. Naturalist, 1893, p. 661. 



Orcodonts with skull structure very similar to that of the John Day genus, 

 Eporeodon, but with molars showing an incipient tendency to hypsodontism. Feet 

 constructed as in Merychyus. Manus "adaptively" reduced, the third metacarpal 

 articulating with the trapezoid. A rudimentary clavicle present. Larynx with 

 ossified thyroid cartilage. 



It may seem that this form is not generically separable from Eporeodon, and the 

 relationship between the two is certainly very close; but if so, it must be given at 

 least subgeneric rank. Animals of this type are much the most abundant fossils of 

 the lower Deep River beds, outnumbering in wealth of individuals all the other 

 species in the collection taken together. Two species are found in association. 



Mesokkodon ohklonti Scott. 



(loc. eit.) 



Size exceeding that of Eporeodon, teeth large, zygomatic arches depressed, occi- 

 put drawn out into supero-lateral wings, metapodials rather short and stout, ungual 

 phalanges trowel-shaped and pointed. 



This species is very much the more abundant of the two and is represented in 

 the collection by a large number of specimens, so that, with the exception of some 

 vertebras and ribs and the sternum, all parts of the skeleton may be described. A 

 considerable degree of variation obtains among these specimens, both in regard to 

 size and in other more important respects. Some of these differences, however, are 

 almost certainly of a sexual nature, and they give no satisfactory reason for establish- 

 ing another species. 



I. Dentition. A. Upper Jaw (PI. Y, Fig. 36). The incisors have small and 

 simple ci-owns, which are antero-posteriorly compressed and in the unworn state are 

 somewhat pointed. In size, they increase regularly from the first to the third. The 

 canine is of the ordinary trihedral recurved shape characteristic of the family, but 

 differs from that of the older genera in having upon its inner face a deep groove, 

 bounded before and behind by sharp enamel ridges. 



The premolars are relatively larger than in Oreodon or Eporeodon and have a 

 different external form, which constitutes an approximation to Merychyus. The 

 change consists in an elongation of the crown, both vertically and antero-posteriorly, 



