OSTEOLOGY 169 



it is probable that many other mammals, fitted for a special 

 sort of agility, are considerably more gifted in these respects. 

 Because of the rigid ankylosis of the tibia and fibula of the 

 genus Neotoma into a single osteological unit, appreciable 

 rotation of the foot at the ankle joint is impossible. Partly 

 due to the great mobihty of the long metatarsus, supination 

 of the anterior plantar surface, though of course not the heel, 

 may be as far as 100 degrees, and what is more surprising, 

 pronatino to 75 degrees from the horizontal is permitted. 

 The reason for this is partly the form of the articular surfaces 

 of the ankle, but there are other reasons as well, not so easy 

 of investigation, as elsewhere discussed. When rotation 

 of the foot is desired, this end is gained by a rotation of the 

 whole leg at the hip. 



Due to the flexion of the toes, the length of the foot is 

 extremely difficult to obtain with exactitude from a skeleton, 

 and one can but compute it with as much accuracy as pos- 

 sible. In Homodontomys the length of the entire foot con- 

 stitutes from 37 to 38 per cent of the functional length of the 

 hind leg; in Neotoma, from 38.8 to 40.8; and in Teonoma 

 it is from 39.6 to 41.4 per cent. (In a single specimen of 

 Neotoma p. pennsylvanica, an animal which seems invariably 

 to live in the crevices of rocky cliffs, this percentage rises to 

 46.5, and in the small Neotoma lepida stephensi, it is but 

 35.2.) 



Tarsus. The tarsus is composed of eight bones. The 

 bone of the ankle joint, upon the dorsum, is the astragalus, 

 articulating dorsad with the tibia and fibula. Its chief de- 

 tails are the articular surface, the neck, and the head ex- 

 tending distad. It also articulates with the calcaneum, 

 scaphoid and tarsal sesamoid bones. To it are attached the 

 quadratus plantae, abductor digiti quinti, and flexor digiti 

 quinti brevis. 



The proximal tarsal bone upon the plantar aspect is the 

 calcaneum, the most massive bone of the foot. Its chief 



