210 ANATOMY OF THE WOOD RAT 



developed in Teonoma, as is the acromial portion of the 

 deltoid, which acts chiefly as a rotator. Other rotators 

 that are larger in this animal are the pectoral mass and 

 teres minor — the latter larger in Neotoma as well. The 

 pectorals act also as an adducting group. The fact that the 

 epicondylar width is greater in Teonoma than in the other 

 two is too complex for satisfactory analysis. 



It is thus seen that the upper arm of Teonoma is the most 

 powerful in practically all motions. Judging strictly from 

 the muscles, Neotoma might be considered as being a trifle 

 stronger, relative to size, than Homodontomys, but its 

 lighter bone casts serious doubt upon this contention. 

 Variations in the myology and osteology of the upper arm 

 do not conform as nicely to our theories as those of some 

 other parts, due, it is beheved, to the complexity involved in 

 the possible amount of movement of which this member is 

 capable. 



The relative length of the ulna in comparison with the 

 functional length of arm is practically the same in the two 

 more scansorial animals, although it is longer in Neotoma. 

 The difi"erence in the length of the radius is very slight, but 

 this bone is definitely hghtest in Neotoma and most robust 

 in Teonoma. There are three ulnar details which are best 

 developed in the latter, and a fourth more marked in this 

 and Neotoma than in Homodontomys. The coracoid process 

 is larger, due to a stronger joint rather than directly to any 

 muscle; the brachial ridge is a trifle sharper, which should 

 be attributable to increased power of either the brachialis 

 or flexor digitorum profundus, or both, not noted during 

 dissection; the tricipital process is larger; and the tricipital 

 fossa is better defined in this and Neotoma. As no difference 

 in the triceps medialis was discerned to account for the last 

 two items, the variation shown by the former at least is 

 probably ascribable to a stronger joint. The epitrochlearis 

 and triceps longus, which act as extensors of the forearm^ 



