116 ANATOMY OF THE WOOD RAT 



of the zygomae is more pronounced in Teonoma, partly to 

 accommodate the greater bulk of temporal muscles passing 

 within them. This detail is less pronounced in Neotoma, 

 and least in Homodontomys. The lesser distance between 

 the posterior portion of the temporal ridges in Teonoma 

 crowds the interparietal to a greater extent, and as age in- 

 creases this exhibits a tendency to change shape from 

 roughly rectangular to more of the form of the section 

 through a truncated cone. 



The braincase or cranium proper is relatively shortest 

 in Teonoma because its cranio-lateral borders do not slope 

 so gradually from the zygomatic arches towards the inter- 

 orbital. Judging by the dorsal aspect of the cranium, one 

 expects the capacity of the braincase to be relatively least 

 in this subgenus, but this seems to be the case to too slight a 

 degree to be of much importance. After stuffing the olfac- 

 tory fossae and the cranial foramina with cotton, the brain- 

 cases of the skulls studied were filled with number ten 

 shot, and the cube root of the number of these was found, 

 so as to render the item more comparable with linear meas- 

 ure. The percentage of this, as compared to the condylo- 

 basilar length, was then computed. The relative difference 

 in brain capacity between the three subgenera is indicated as 

 being rather insignificant (about 18.2 per cent for Neotoma, 

 17.7 for Homodontomys, and 17 for Teonoma). It is thus 

 seen that Teonoma does have a smaller capacity, although 

 to a slighter degree than a visual inspection suggests. 



According to the above figures, Teonoma has the smallest 

 braincase, relative to length of skull, and Neotoma the 

 largest. The latter circumstance is unexpected, although 

 with a larger series the positions of Homodontomys and 

 Neotoma might be reversed in this respect, for, it seems to be 

 a fact, broadly speaking, that mammals with the largest 

 temporal muscles have the smallest brains (see Anthony, 

 1903), and that a more extensive muscular covering in- 



