284 



ANxVTOMY OF VERTEBRATES. 



of the palatines enters into the formation of the jDOsterior 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 j)erforated lengthwise 

 by the ectocarotid. The crown of from one to three of the hinder 

 premolars is as complex as those of the molars : ^ that of the last 

 lower milk-molar is commonly bilobed. To these osteological and 



165 



Perissodactyle skeleton {Rhinoceros Indicus). Lxxiii 



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 Perissodactyle hoofed quadrupeds, and their claims 

 to be regarded as a natural group of the Ung2data. 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, wdth decidua, as in the 

 Elephant. But the diffused placenta occurs in some genera of 

 the next group, showing the inapplicability of that character to 

 exact classification. The oldest known Perissodactyles are from 

 the lowest tertiary strata. Many extinct genera, e.g. Coryphodon, 

 Plioloplius, Lophiodon, Tapir other ium, PalcEotherium, Ancitherium, 

 Hijjparion, Acerotlieriiim, Elasmotlierium, &c., have been dis- 

 covered, which once linked together the now broken series of 



' Some early tertiary extinct forms (Pliolophus, Coryphodon, Lophiodon') oifcred 

 exceptions to this rule. 



