DISCUSSION 209 



brevis and coracobrachialis, originating therefrom; but no 

 difference was noted in these muscles during dissection. 



The functional length of arm in Homodontomys averaged 

 44.4, in Neotoma 49, and in Teonoma 47.5 per cent of their 

 respective body lengths. It is unfortunate that we can not 

 know whether this is actually due to variation in the length 

 of the limb or of body, for there is no basic standard with 

 which the subgeneric body lengths may be compared. 



The humerus, as well as most of the bones of the extremi- 

 ties, is heaviest proportionate to length in Teonoma and 

 lightest in Neotoma. It is also a trifle the longest in the 

 former and shortest in the latter. The larger size of the 

 humeral head in Teonoma is correlated only with the 

 greater size of the bone as a whole. The teres minor is 

 indicated as being a trifle less efficient in Homodontomys 

 but this apparently has no efiect upon the bone. The 

 better definition of the proximal portion of the medial ridge 

 in Teonoma seems to be due to the more triangular shape, 

 and presumably greater strength, of the abdominal pectoral, 

 and possibly also to slight differences in the latissimus dorsi. 

 The deltoid spine was noted as averaging a trifle higher in 

 Neotoma than the other two animals, but there seems to 

 be no myological reason for it. In fact the acromial portion 

 of the deltoid muscle, which might influence its height, was 

 found to be best developed in Teonoma. The increased 

 length in Teonoma of the deltoid ridge or rather the dis- 

 tinctness of its more distal portion is a reflection of the 

 larger size in that animal of the superficial and deep pectorals 

 acting as rotators and adductors, and the posterior division 

 of the latter to some extent as a flexor, of the brachium. 

 It is likely that their lever arm is slightly greater as well. 

 Neither the lighter panniculus carnosus of this subgenus 

 nor the heavier one of Neotoma seems to have caused appre- 

 ciable variation at the point of insertion. The latissimus 

 dorsi and triceps longus (flexors of the humerus) are best 



