SKELETON OF PERISSODACTYLA. 



451 



the formation of the palato-nares. The lacrymal canal commences 

 by one or two foramina, defended by a process. The maxillary 

 forms the floor of the orbit, as in the Rhinoceros and Tapir ; but 

 the premaxillaries join the nasals. The lower jaw, 7, is remark- 

 able for the backward expanse of the ascending ramus. The 

 coronoid process is perforated lengthwise at its base. 



If the equine skull, fig. 303, be compared with that of the 

 Rhinoceros, the basioccipital will be seen to be narrower and 

 more convex. The mastoid, 8, intervenes, as a tuberous process, 

 between the post-tympanic and paroccipital processes, clearly 

 indicating the true nature of the post-tympanic in the Rhino- 

 ceros ; the Tapir shows an intermediate condition of the mastoid 



303 



Skuil of Horse, Equus CabalUis. 



between the Rhinoceros and Horse. The latter differs from both 

 the Tapir and Rhinoceros in the outward production of the roof 

 of the orbit and the completion of the bony frame of that cavity 

 behind by the junction of the postorbital process, i, 2, c, with the 

 zygoma, ?-: Equus resembles Macrauchenia in this particular. 

 The temporal fossa, 7, is small in proportion to the length of the 

 skull : the base of the postorbital process is perforated by a 

 superorbital foramen, h : the lacrymal canal begins by a single 

 foramen. The premaxillaries, 22, extend to the nasals, 15, and shut 

 out the maxillaries, 21, from the anterior aperture of the nostrils. 

 The chief marks of affinity to other Perissodactyles are seen in 

 the shape, size, and formation of the posterior a])crture of the 

 nostrils, the major part of which is bounded by the palatine bones, 



o G 2 



