OSTEOLOGY 117 



hibits by just so much the growth of the bones concerned. 

 In this respect it is significant that the largest Teonoma 

 among the large series available, with condylo-basilar 

 length of 54.6 mm. and temporal ridges but 3.6 mm. apart, 

 has a brain capacity of but 14.7 per cent, figured by the 

 above formula. 



Turning to the ventral aspect of the skull, it is found that 

 Teonoma has the longest diastema in comparison to its 

 condylo-basilar length (34.2 per cent), and Neotoma the 

 shortest (30.2, with Homodontomys 31.3). One point influ- 

 encing the length of the diastema in the last is the fact 

 that either the upper molar row is situated more caudad 

 than in the other two, or what is more likely, the zygomatic 

 processes of the maxillae are located a trifle more craniad. 

 There is very slight relative difference in the length of the 

 incisive foramina compared to the extent of the diastema. 

 The nasal arteries and nasopalatine nerves passing through 

 these foramina are small, and hence the large size of the 

 foramina and their variation in different mammals seems 

 due entirely to Jacobson's organs, of the nasal passages. 



The zygomatic processes of the maxillae, in comparison 

 with the posterior portion of the zygomatic arches, are rela- 

 tively broadest in Homodontomys, less so in Neotoma, and 

 least in Teonoma. Another fact of significance is that in the 

 last-mentioned genus the bony septum of the zygomatic 

 process of the maxilla is narrowest, while in the other two it 

 is about equal in width. These circumstances indicate that 

 at least the greater portion of the masseter muscle is stronger, 

 or at least relatively larger, in Homodontomys, less so in 

 Neotoma, and least in Teonoma. Osteological differences 

 in the posterior portion of the zygomatic arch are not of 

 such a character as to enable one to judge regarding any 

 subgeneric variation in the development of the fibers of the 

 muscles originating therefrom. 



There is much individual and some specific difference in 



