HISTQEY OP THE PERISSODACTYLA 



289 



among the true rhinoceroses of the Old and New Worlds, 

 which long followed parallel, but quite independent, courses 

 of development, and five phyla 

 among the American horses. While 

 these phyla add so much to the dif- 

 ficulty of working out the genealogi- 

 cal series, it is possible to simplify 

 the problem and treat it in a broad 

 and comprehensive manner that 

 will sufficiently estabhsh the essen- 

 tial steps of change. 



In external appearance and gen- 

 eral proportions the different fami- 

 lies of existing perissodactyls.have 

 very Uttle in common ; "that tapirs 

 and rhinoceroses should be related 

 is not surprising, but the horses 

 would seem to be as far removed 

 from both of the former as possible. 

 Why, then, should they be included 

 in the same order ? A study of the 

 skeleton, however, reveals the com- 

 munity of structure which obtains 

 between the three famiUes, a com- 

 munity which removes them widely 

 from all other hoofed manunals. In 

 all existing perissodactyls, though 

 not in most of the Eocene genera, aU 

 the premolars, except the first, have 

 the size and pattern of the molars. 

 The foramina of the skull, or per- 

 forations by which blood-vessels and nerves enter and leave the 

 cranium, are arranged in a way characteristic of the order and 

 different from that seen in other hoofed mammals. The femur 

 always has the third trochanter. The number of digits in each 



Fig. 145. — Left manus of Tapir 

 (Tapirus terrestris). S., scaph- 

 oid. L., lunar. Py., pyramidal. 

 Pis., pisiform. Td., trapezoid. 

 M., magnum. Un., unciform. 

 The metacarpals are erroneously 

 numbered. Mc. I., second met- 

 acarpal. Mc. II., third do. 

 Mc. III., fourth do. Mc. IV., 

 fifth do. Ph. 1, first phalanx. 

 Ph. 2, second do. Ung., un- 

 gual phalanx. 



