318 New Species of Aplodontia. 



larger than that of A. rufa, the teeth have remained of the same 

 absolute size, the differences noted being merely those of indi- 

 vidual variation. Thus the ratio of the molar series (measured 

 on the crowns) to the basilar-length is 26.20 in the largest skull 

 of A. major, and 28.59 in the largest of A. rufa. 



The occipital crest is much more pronounced in A. major, 

 giving the posterior aspect of the cranium a contour quite dif- 

 ferent from that of A. rufa. The greater height of the occipital 

 crest in A. major is relative as well as absolute, the ratio of its 

 vertical height from the apex of the paroccipital process in an 

 adult male being 37.09, while in the largest male of A. rufa this 

 ratio is but 29.38. 



The zygomatic arches furnish some of the best cranial charac- 

 ters in distinguishing the two species. The jugal bones are not 

 only twice as thick and heavy in A. major as in A. rufa, but 

 they are farther apart anteriorly,* and the arches are more bowed 

 outward, giving the skull a very different outline. Moreover, 

 the vacuities which they enclose differ in shape and relative size 

 in the two species. Viewed from above, they are shorter and 

 broader in A. major ; measured from below, their absolute length 

 is the same in both, notwithstanding the fact that the skull of 

 A. rufa is so much smaller. Hence the ratio of the greatest 

 length of the zygomatic vacuity to the basilar-length is 37. G8 in 

 A. major and 39.63 in A. rufa. 



The post-zygomatic notch is a trifle narrower and deeper in 

 A. major than in A. rufa. 



The length of the frontal and parietal together, and the ratio 

 of this length to the basilar-length, are considerably greater in 

 A. major than in A. rufa. 



The length of the ascending or frontal process of the premax- 

 illary is about the same in the two species, notwithstanding the 



* This greater breadth across the maxillary roots of the zygomae affects 

 somewhat the position of the ant-orbital foramina. They are set out farther 

 from the muzzle in A. major than in A. rufa. The average of the least dis- 

 tance between them in the largest four skulls of A. major is 18.47 mm. ; 

 while in the three largest of A. rufa it is 16.03 mm. The ratio of these ave- 

 rages to averages of the basilar-length in these same skulls, is 27.01 in A. 

 major and 24.92 in A. rufa. 



