120 



CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



case than those of P. (Phenacomys) intermedins, much as in 

 P. (Phenacomys) orophilus. The narrower interorbital con- 

 striction is the most certainly diagnostic character of the crania 

 of members of the new subgenus (see table of measure- 



A Cranial measurements comparative of 

 Phenacomys (Arborimus) longicau'dus and Phenacomys (Phenacomys) 



orophilus 



(All measurements in millimeters) 



Width of 

 cranium 

 outside 

 external 

 Greatest Zygomatic Interorbital auditory- 

 Museum length width constriction^ meatus 



No. Sex Phenacomys orophilus 



205916 $ 25.8 15.1 3.5 12.0 



109103 S 26.6 3.9 12.2 



67327 S 25.7 14.9 3.6 11.7 



110249 ? 25.0 14.9 3.8 11.5 



Phenacomys longicaudus 



21145 S 24.4 14.3 3.0 11.1 



21148 S 24.4 13.9 3.4 11.6 



19983 9 26.1 14.6 3.3 12.4 



19174 9 25.0 15.1 3.4 11.9 



21149 9 25.0 14.2 3.2 11.6 



21143 9 25.2 14.4 ' 3.2 12.4 



19973 9 25.1 14.1 3.3 12.2 



19130 9 24.6 14.3 3.4 11.5 



19984 9 25.8 14.9 3.5 12.4 



iMiller remarks (1897, p. 80) that the breadth of the interorbital region in Phen- 

 acomys is a character of trifling importance, which might easily disappear with in- 

 creasing age. But the character as a good diagnostic feature would seem to hold 

 between the series before me, since the specimens compared are good adults, with the 

 examples of orophilus averaging older than those of longicaudus. 



ments). Palate posteriorly tends to be different in Arborimus, 

 usually having lateral pits a little deeper, sloping portion of 

 median ridge longer, and lateral bridges more often present. 

 In Phenacomys the lateral pits tend to be shallower, the sloping 

 portion of the median ridge is shorter, in some specimens 

 almost obsolete, and the lateral bridges are more often absent 

 than present. There is overlapping between the subgenera in 

 this respect. Young individuals of longicaudus tend to resemble 

 orophilus and intermedins. 



Enamel pattern in Phenacomys longicaudus simpler than in 

 certain representatives of the subgenus Phenacomys, but not 

 presenting any characters in all cases diagnostic of the new 

 subgenus. The first and second upper molars are practically 



