39 



There is a well-developed post-tympanic process which is 

 separated by the true mastoid from the paroccipital in the 

 Horse, but unites with the lower part of the paroccipital in 

 the Tapir, and seems to take the place of the mastoid in the 

 Rhinoceros and Hyrax. The hinder half, or a larger propor- 

 tion, of the palatines enters into the formation of the posterior 

 nares, the oblique aperture of which commences in advance 

 either of the last molar, or, as in most, of the penultimate one. 

 The pterygoid process has a broad and thick base and is per- 

 forated lengthwise by the ectocarotid. The crown of from one 

 to three of the hinder preinolars is as complete as those of the 

 molars : that of the last lower milk-molar is commonly bi- 

 lobed. To these osteological and dental characters may be 

 added some important modifications of internal structure, as, 

 e. g., the simple form of the stomach and the capacious and 

 sacculated caecum, which equally evince the mutual affinities 

 of the odd-toed or perissodactyle quadrupeds with hoofs, and 

 their claims to be regarded as a natural group of the UNGULATA. 

 Many extinct genera, e. g. Coryphodon, Plioloplius, Lopldodon, 

 Tapir other ium, Palceotlierium, Ancitherium, Hipparion, Acero- 

 therium, Elasmotherium, &c., have been discovered, which 

 once linked together the now broken series of Perissodactyles, 

 represented by the existing genera Rhinoceros, Hyrax, Ta- 

 pirus, and Equus. The placenta is replaced by a diffused 

 vascular villosity of the chorion in all the recent genera of this 

 order, excepting the little Hyrax, in which there is a localised 

 annular placenta, as in the Elephant. But the diffused pla- 

 centa occurs in some genera of the next group, shewing the 

 inapplicability of that character to exact classification. 



In the even-toed or ' artiodactyle ' Ungulates, the dorso- 

 lumbar vertebras are the same in number, as a general rule, 

 in all the species, being nineteen. The recognition of this 

 important character appears to have been impeded by the 

 variable number of moveable ribs in different species of the 

 Artiodactyles, the dorsal vertebras, which those ribs charac- 

 terize, being fifteen in the Hippopotamus and twelve in the 



