No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 339 



beyond its Bridger predecessor, Pachynolophus, much less in 

 regard to size than in morphological differentiation ; the larger 

 species of the Bridger genus are but little inferior in this respect 

 to the smaller species of the White River form. In spite of its 

 comparatively high degree of differentiation, Mesohippus was a 

 very small animal compared with the recent horses, about the 

 size of a Newfoundland dog. The skeleton is essentially like 

 that of existing Eqitidce in character and appearance, but pre- 

 sents many striking points of difference. 



The skull is much smaller, and in particular the facial region 

 is shorter, shallower, and more tapering. The orbit is relatively 

 very large, not enclosed behind, and situated very far forward 

 and low down in the face. This position of the orbit brings 

 about other noticeable differences in the character of the skull, 

 as for example the great elongation of the zygomatic arch, and 

 especially of the malar or jugal portion of it. The arch is also 

 quite slender and the masseteric crest is but slightly developed 

 and does not extend forward upon the maxillary. The short- 

 crowned molars render the alveolar portion of the maxillary 

 very low and contracted, in sharp contrast to the alveolus in 

 those genera which have prismatic teeth, and owing to the very 

 small size of the canine and incisor teeth, the premaxillaries 

 and the symphysis of the lower jaw are short and slender, which 

 in its turn modifies the shape and size of the anterior narial 

 opening. The ascending ramus of the mandible is relatively 

 low and of quite a different shape from that seen in Eqaus, and 

 the condyle is not placed nearly so far above the level of the 

 molars. 



The neck is quite long, but the vertebrae are much less mas- 

 sive, not so decidedly opisthocoelous, and with less strongly 

 developed processes than in Equus. The neural spine and 

 odontoid process of the axis are of entirely different shape in the 

 two genera, and in the Miocene form the spines on the fifth 

 and sixth vertebrae are much higher. The back is long, a-i, 

 as appears from the faces of the posterior dorsal and lumbar 

 vertebrae, much more arched than in the horse ; the spines, so 

 far as they are preserved, are more slender and compressed. If 

 we may judge from the comparatively few fragments which have 

 been preserved, the ribs were more slender, rounded, and less 

 flattened than in the horse, — a peculiarity which is common to 

 nearly all of the earlier ungulates. 



