854 PHYLUM CHORDATA : CLASS MAMMALIA 



galus has a pulley-like surface above for articulation with 

 the tibia ; its distal surface is flattened and unites to a much 

 greater extent with the navicular than with the cuboid. 

 The last-named bone is of less importance than in the 

 Artiodactyla. The calcaneum does not articulate with 

 the lower or distal extremity of the fibula. The femur 

 has a third trochanter or process for the insertion of 

 muscles. There are usually twenty-three dorso-lumbar 

 vertebrae. 



As to the dentition, the premolars and molars form a 

 continuous series, with broad transversely ridged crowns, 

 the last premolars often very like the molars. 



The stomach is simple ; the caecum is large ; there is no 

 gall-bladder. 



The mammae are inguinal ; the placenta is diffuse and 

 non-deciduate. 



Families of Perissodactyla 



Family Tapiridae. — In the Tapirs {Tapirus) there are four digits in 

 the manus, but the third finger is still practically median, as the 

 fifth digit scarcely reaches the groimd. The hind-foot has three 



digits. The dentition of the genus is ?-^. The orbit and 



^ ^ 3133 



temporal fossa are continuous. The nose and upper lip form a 



short proboscis. The thick skin has but scanty hair. In habit, 

 the tapirs are shy and nocturnal, fond of forests and water, 

 feeding on tender shoots and leaves. The distribution is some- 

 what remarkable, for four species live in Central and South 

 America, while a fifth is Malayan. The genus was once 

 widespread, but has survived in these two far-separated regions. 

 Family Equid®. — In the modern horses (Equus) there is on each 

 foot one functional digit — the third, with splints representing the 

 m.etacarpals and metatarsals of the second and fourth. Professor 

 Cossar Ewart has demonstrated in the embryo of the horse the 

 rudiments of the three phalanges of the second and fourth digits. 

 The vestigial phalanges of these digits subsequently fuse with 

 one another and with the respective metacarpals or metatarsals, 

 forming " buttons " at the end of the splints. The ulna and 

 fibula are incomplete, but the former is quite complete in the 



foetus. The dentition is H13 ]-,ut ^^^ first premolar is rudi- 



3143 

 mentary, and soon lost in both sexes, and the canines are rarely 



present in the mare. The orbit is complete. 



The modern horses are connected by a very complete series of forms 



with ancestral Eocene types, but it is not clearly proved that these 



forms were actually in the line of descent of the genus Equus. The 



