SKULL — MAMMALS 215 



includes the foramen lacerum in front and the jugular behind the tympano- 

 petrosal. In some rhinoceroses postglenoid and posttympanic processes unite 

 in such a way that an opening, the false auditory meatus, is formed. The 

 mastoid is partly visible in horses, not in others. The lower jaw has a high 

 ramus, the coronoid process is slender, the condyle short, permitting of slight 

 lateral motion. The symphysis of the two halves of the jaw is long in tapirs. 

 The hyoid apparatus (fig. 209) is well ossified and has a long stylohyal connected 

 with the tympanohyal. 



Associated with the Perissodactyls are a number of extinct groups as well as 

 the existing Hyracoidea, Proboscidea and Sirenia, which are retained here as 

 separate orders. Of the fossil groups the following deserve mention. 



CoNDYLARTHRA have skuUs recaUing Carnivores and Ungulates, and were 

 regarded by Cope as ancestral to the whole group of Ungulates in the wider sense, 

 as well as to other modern forms. The cranium has a small brain case, with a low 

 and long sagittal crest, a slender zygomatic arch; orbits and temporal fossa 

 scarcely separated. The nasals reach back between the orbits, the premaxillse 

 are weak and the mandibular fossa has a postglenoid process. Both foramen 

 rotundum and alisphenoidal canal are present. 



LiTOPTERNA.— In some cranial features these fossils resemble the Perisso- 

 dactyls, differ in others. The long cranium with small cavity lacks a crest, the 

 zygomatic arch is complete, the orbit (except in Thesodon) is separate from the 

 temporal fossa. The nares are sometimes nearly normal, but in Macrauchenia 

 are at about the middle of the dorsal surface, a position which, with the deep pits 

 on the anterior parts of the frontals, suggests the presence of a proboscis some- 

 thing like that of the elephant. The nares, which are vertical as in whales, 

 have large nasals which, except in size, recall those of Sirenia, a resemblance 

 heightened by the frontals. The lacrimal, with from one to three foramina, has 

 a large facial part. 



A^XYLOPODA. — This order, largely based on foot structure, falls into three 

 groups, so far as skulls are concerned. The Taligrada have non-pneumatic 

 crania, a sagittal crest, premaxillse extending back to the nasals which lie between 

 the orbits; and are without horns or marked protuberances on the roof. The 

 small orbits are in widest connexion with the temporal fossae, the coronal suture 

 persists. There is no alisphenoidal canal. The ramus of the lower jaw is high 

 and the condyle faces upwards. 



Pantodonta and Dinocerata have pneumatic bones, no sagittal crest, 

 the coronal suture is obliterated and there are knobs on the frontals. The 

 Pantodonta have premaxillse which fall short of the frontals, no protuberances 

 on the maxillae, rudimentary horns on the parietals; nasals (lacking horns) 

 between the orbits and no alisphenoidal canal. The mandibular ramus is high 

 and the condyle looks obliquely. In Dinocerata nasal and premaxilla meet, 

 small prenasals being present, the nasals short and not reaching the orbits, 

 but bearing horns, while maxillae and parietals have large horns. An alisphe- 

 noidal canal occurs, the ramus of the lower jaw is low and the condyle faces 

 backwards. All Ancylopda agree in the complete zygomatic, and have at most 

 but a slight postorbital process. 



