226 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



The outer triangle in longicaiidus tends to be of compara- 

 tively small size and not tightly closed off from the antero- 

 internal triangle and the opposed inner triangle. In most 

 specimens the outer triangle opens rather broadly into the 

 opposed inner triangle and narrowly, if at all, into the antero- 

 internal triangle. In intermedins the outer triangle is larger 

 than in longicaudus, and is closed off in both directions. In 

 orophilus there is a considerable range of individual variation 

 in this respect. In four adult examples (Nos. 109103, 67327, 

 110249, and 205916) the outer triangle is large and is closed. 



According to Merriam's plate IV (1889, opp. p. 44) the 

 outer triangle in latimanns opens very narrowly anteriorly. 



Summarizing, it may be said that while in the subgenus 

 Phenacomys the antero-external loop is often closed, it never 

 is in Phenacomys (Arhorimus) longicaudus; there is also 

 apparent a tendency in the latter for the outer triangle to be 

 smaller and more open than in the subgenus Phenacomys. 



(5) Third lower molar 



Typical of the genus, being made up of three transverse 

 triangular digitations connected along the outer border of the 

 tooth. Tending to be simpler in P. longicaudus than in the 

 subgenus Phenacomys, in which there is often if not usually 

 a small outer triangle pinched off opposite the middle trans- 

 verse triangular digitation. 



In one specimen of longicaudus (No. 21150) this little outer 

 triangle is definitely outlined, though it is not closed off. 

 Similar conditions obtain in several other specimens. There is 

 considerable variation in orophilus. In two specimens (Nos. 

 31249 and 75029) no triangle is outlined, and this is true in 

 several young examples also; but in most adult examples the 

 little triangle is emphasized and tightly closed. In intermedius, 

 type of the genus, the triangle is well developed, being practi- 

 cally closed in Nos. 174431 and 174432, but opening posteriorly 

 in the right molar of No. 174425. 



Summarizing, in longicaudus an outer triangle on the third 

 lower molar is never isolated in the sense of being tightly 

 closed. In the subgenus Phenacomys there is a tendency for 

 an outer triangle to be isolated in this manner. 



