OSTEOLOGY 149 



Forearm 



The antibrachium consists of two bones, the ubia being 

 the longer and the radius the shorter. 



Ulna. In Homodontomys the ulna averages 58.3 (56.8 to 

 60) per cent of the functional arm length; in Neotoma it 

 averages 60 (59.2 to 61) per cent; and in Teonoma, 57.9 

 (56.2 to 59) per cent. It is thus seen that in the animal 

 which is presumably the poorest climber, the ulna, and 

 therefore the forearm, averages appreciably longer with 

 respect to body length than in the other two. As previously 

 mentioned, this naturally accompanies a correspondingly 

 short humerus. 



The ulna, like most of the other bones, is least robust in 

 Neotoma. Upon the cranial aspect near its proximal end is 

 situated the greater sigmoid cavity, by which articulation 

 with the humerus is effected. Upon the distal border of this 

 occurs the coronoid process. Laterad upon the border of the 

 latter there is a small facet for the articulation of the radius, 

 and mediad the process is slightly extended, this marking the 

 origination of the flexor digitorum sublimis. It is least ac- 

 centuated in Neotoma and somewhat the best defined in 

 Teonoma, as might be expected, for the muscle is larger in 

 the latter animal and divides into four, instead of three 

 tendons. Proximad of the sigmoid cavity occurs the ole- 

 cranon, which plays an important part in the economy of the 

 animal, for upon this extension of the forearm largely de- 

 pends the leverage which that member can exert. Its ex- 

 tent proximad from the sigmoid cavity is from 11.3 to 13.3 

 per cent of the ulnar length. Laterad upon the proximal 

 border of the sigmoid cavity is the tricipital process, perhaps 

 a trifle the best defined in Teonoma. The triceps mediahs 

 is inserted upon this and the fossa immediately proximad, 

 which is a bit the least distinct in Homodontomys. Caudad 

 to this along the caudo-lateral border is the tricipital ridge, 



