OSTEOLOGY 147 



carnosus and pectoralis profundus anterior, the latter ex- 

 tending upon the greater tuberosity; along the same border 

 of the distal ridge are inserted the pectorali superficialis and 

 profundus posterior. Inserted chiefly upon the spine itself 

 are the clavo-acromiodeltoid, and upon both the spine and 

 the adjoining ridges the spinodeltoid and brachialis. There 

 are thus seven muscles concerned with this feature of the 

 humerus. The panniculus is the only one of these that is 

 best developed in Neotoma, but the superficial pectoral is also 

 more extensive than in Homodontomys, and in this animal the 

 deltoid spine, but not the remainder of the deltoid ridge, is 

 highest and most distinct. On the other hand, the pec- 

 torali superficialis and profundus posterior, and the clavo- 

 acromiodeltoid seem to be best developed in Teonoma, and 

 in this subgenus the deltoid spine as a whole is most exten-- 

 sive, although the spine is not as distinct as in Neotoma^ 

 The distance from the humeral head to the most prominent 

 point of the spine is from 35 to 41 per cent of the length 

 of this bone, the larger percentages usually occurring in 

 Teonoma. 



The origin of the triceps medialis extends from the process 

 of the medial shaft along practically the entire olecranol 

 fossa, but there is no variation in this part of the humerus. 

 The same is to be said regarding the distal portion of the 

 origin of the brachialis, distad from the deltoid ridge. 



The distal termination of the humerus is characterized by 

 the medial and lateral epicondyles. The greatest width of 

 the distal portion of the humerus averages 23.8 per cent of its 

 length in Homodontomys, 23.7 in Neotoma and 24.5 in 

 Teonoma. The lateral epicondyle is the culmination of the 

 very prominent epicondyloid ridge. This varies practically 

 not at all subgenerically, but the individual difference is 

 great. It may project a couple of millimeters laterad of 

 the epicondyle or not at all. Origins of the extensores carpi 

 radiahs longus and brevis and digitorum communis are most 



