360 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



at a glance. The calcaneum, or heel-bone, has a large convex 

 facet, by means of which it articulates with the fibula, or exter- 

 nal leg-bone ; there is no such articula- 

 tion in the perissodactyls. The lower 

 end of the calcaneum is narrow and fits 

 into a step cut in the cuboid, which 

 IS thus every whit as peculiar and char- 

 acteristic as the calcaneum and astrag- 

 alus. The femur never has the third 

 trochanter, which is always present in 

 the perissodactyls. Another respect in 

 which the artiodactyls differ from all 

 perissodactyls except the horses is in 

 the much more complex mode of articu- 

 lation between the vertebrae of the lum- 

 bar and posterior dorsal regions, which 

 the former display, and even the horses 

 have no such elaborate arrangement. 

 Finally, another very marked difference 



M. Jr. 



Fig. 188. — Left pes of Pig. 

 Cal., calcaneum. As., as- 

 tragalus. N., navicular. 

 C6., cuboid. Cn. 2, Cn. 3, 

 second and third cunei- 

 forms. Mt. II-V, second 

 to fifth metatarsals. 



Fig. 189. — Bunodont 

 upper molar of pec- 

 cary (Tagassu). 



Fig. 190. — Selenodont 

 upper molar of deer 

 {Odocoileus) . 



from the perissodactyls is in the teeth, for the premolars and 

 molars are never alike, and only in very rare instances does 

 the last premolar assume the molar-pattern. Of this pattern, 

 there are two principal kinds, one exemplified by the pec- 

 caries, in which the crown supports a series, fundamentally 

 two pairs, of conical cusps, and called bunodont, and the 

 other, to be seen, in all the ruminating animals, in which the 

 crown is composed of two pairs of crescents and is therefore 

 said to be selenodont. The bunodont was the primitive type, 



