DISCUSSION 207 



definitely stronger in Teonoma, less so in Neotoma, and least 

 in Homodontomys — for to just what purpose it is difficult 

 to state. The omo- and stylohyoids are larger in both 

 Neotoma and Teonoma, and the stylohyoid largest in the 

 latter animal. These muscles help to move the hyoid bone 

 and assist in the act of deglutition; hence we are tempted 

 to infer that they are larger as an aid to the presumably 

 more difficult swallowing of the more fibrous fare of Neo- 

 toma and Teonoma. 



The thoracic series of vertebrae is relatively longest in 

 Neotoma and but little shorter in Teonoma. In the lumbar 

 series the opposite sequence is found, that of Homodontomys 

 being longest, and of Neotoma, shortest. The subgeneric 

 differences in the relative length of the sacral vertebrae are 

 very slight, but they undoubtedly tell an adaptational 

 tale had we sufficient data accumulated to understand it. 

 The reasons causing subgeneric variation in the nine other 

 vertebral details tabled can not be attributed to any definite 

 muscle differences, although they have undoubtedly been 

 caused, at least indirectly, by myological stimuU. The 

 back muscles are always extremely complex in the precise 

 attachments of their fibers. The only muscular differences 

 in this region noted during dissection were that the iUocos- 

 talis, which is a lateral bender or stiffener of the back, 

 seemed to be smaller in Teonoma, and the psoas minor, a 

 vertical flexor of the lower back, was larger in both Teo- 

 noma and Neotoma. 



Relative to body length the caudal series of vertebrae of 

 Teonoma is 18 per cent, and of Neotoma 11 per cent shorter 

 than that of Homodontomys. In Teonoma the long hairs of 

 this member, assisted by the larger extensor caudae mediahs, 

 make up in its efficiency as a balancer for its shorter length. 



Variation in the sternum, which is lightest in Neotoma and 

 on the whole most robust in Teonoma, can not be correlated 

 with muscular or functional facts, with any degree of cer- 



