DIVISIONS OF PERISSODACTYLES 



237 



complete one, the more modern forms as usual being the more 

 deficient in numbers of teeth. 



The dorso-lumbar vertebrae are as a rule twenty-three ; but 

 the extinct Titanotheres are again an exception ; for, at least in 

 Titanotherium, there are but twenty of these vertebrae an Artio- 

 dactyle character. The femur has a third trochanter. There are 

 so few recent Perissodactyles that an enumeration of the dis- 

 tinguishing characters of the viscera may very probably be use- 

 less for purposes of classification. But 

 the living genera at any rate are to be 

 separated from the living Artiodactyles 

 by the invariable simplicity of the * 

 stomach coupled with a very large and 

 sacculated caecum. The liver is simple 

 and not much broken up into lobes, and 

 the gall-bladder is always absent. The 

 brain is well convoluted. The teats are 

 in the inguinal region. The placenta in 

 this group is of the diffused kind. 



The living Perissodactyles belong to 

 three types only, indeed to three genera 

 only (in the estimation of most), which 

 are the Horses, Tapirs, and Rhinoceroses. 

 But taking into account the extinct 



forms, they may be divided primarily Fia 123. -Anterior aspect of 

 (according to. Professor Osborn) into the 

 four following groups : (1) Titano- 

 therioidea, including but one family, 

 Titanotheriidae ; (2) Hippoidea, includ- 

 ing the families Equidae and Palaeotheriidae ; (3) Tapiroidea, 

 with two families, Tapiridae and Lophiodontidae ; and (4) Ehino- 

 cerotoidea with families Hyracodontidae, Amynodontidae, and 

 Ehinocerotidae. It is conceivable, according to the same writer, 

 that the Chalicotheres (here treated of as a separate sub-order, 

 Ancylopoda) should be added to the Perissodactyle series. 



Fam. 1. Equidae. This family, which includes the living 

 Horse, Zebras, and Asses, as well as a number of extinct genera 

 agreeing with those types in structure, may be defined by the 

 possession of but one functional toe, the two lateral ones being mere 

 splints, or but little more. The molar teeth are hypselodont, and 



right femur of Rhinoceros 

 (Rhinoceros indicus). x A. 

 h, Head ; t, great trochanter ; 

 t', third trochanter. (From 

 Flower's Osteology.) 



