No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 309 



existing type. The masseter crest is very feebly developed, and 

 does not extend at all in advance of the jugal suture. 



The palatines form very little of the hard palate and are 

 chiefly confined to enclosing the posterior nares laterally. This 

 opening is decidedly horseshoe-shaped, broad and rounded 

 in front, and much contracted behind. The palatines are in 

 contact with the maxillaries throughout, the inner wall of the 

 orbit being formed in its postero-inferior portion by the outer 

 wall of the narial opening, giving to this region of the skull an 

 entirely different appearance from that which occurs in the 

 horse. The pterygoids are very small and inconspicuous, and do 

 not form distinct hamular processes. The vomer, so far as it 

 can be made out, agrees v/ith that of Equns. 



The inferior maxillary has a slender and compressed horizontal 

 ramus, which tapers rapidly forward, and is very sharply con- 

 stricted at the diastema, expanding again slightly to form the 

 incisor alveolus. The two rami are firmly co-ossified, and the 

 symphysis is quite long. The symphysial region is directed 

 quite sharply upward and forward, thus forming an angle with 

 the remainder of the horizontal ramus, while in Equus the sym- 

 physis is flattened, procumbent, and in the same straight line 

 as the molar-bearing region. These differences are no doubt due 

 to the changes in the character and especially the size of the 

 incisor teeth. The angle is regularly rounded and has a thick- 

 ened border ; it is more extended antero-posteriorly than in the 

 horse. The ascending ramus, on the other hand, is decidedly 

 lower, and the condyle less elevated above the level of the 

 molars. The coronoid process is regularly recurved and pointed, 

 and resembles that of the ruminants rather than that of Equus. 

 The condyle is extended transversely, but very narrow antero- 

 posteriorly. 



The cranial foramina of Mesohippus have attained very nearly 

 the modern arrangement. The anterior palatine foramina are 

 much shorter than in the horse, and the posterior palatine fo- 

 ramina are farther forward, opposite the first molar. The infra- 

 orbital is over pm. 3, as in Equus, but much lower down than in 

 that genus, and it is separated from the orbit by a much shorter 

 interval. The supra-orbital foramen is sometimes present, some- 

 times indicated by a notch. The optic foramen is placed in front 

 of the foramen lacerum anterius, and not above it. There is an 



