Vol. V] TAYLOR— NEW SUBGENUS GF PHENACOMYS \2\ 



identical with those in the subgenus Phcnacomys, and will not 

 be discussed separately. The front upper molar has, beginning 

 anteriorly, a transverse loop, two inner triangles, one oiiter 

 triangle, and a postero-external loop. The middle upper molar 

 has transverse loop, one outer triangle, one inner triangle, and 

 a postero-external loop. 



(2) Back upper molar 



Pattern practically as in the subgenus Phenacomys. Begin- 

 ning anteriorly, the tooth shows a transverse loop, an external 

 triangle, an internal triangle, a second outer triangle, and a 

 postero-internal loop. 



In the subgenus Phenacomys the outer portion of the pos- 

 terior "trefoil" ordinarily opens into the inner portion, forming 

 a figure crescentic or boomerang-shaped in outline, the con- 

 cavity of the crescent or boomerang being directed forward. 

 Sometimes the crescent or boomerang is symmetrical, but 

 usually the inner arm is the heavier. In Phenacomys longi- 

 candus the outer portion of the crescent tends to be closed off 

 from the inner portion, forming a second triangle externally 

 and a loop internally, the loop tending to be more rounded 

 than is ordinarily the case in the subgenus Phenacomys. 



12 3 4 



Fig. 1— ENAMEL PATTERN OF BACK UPPER MOLARS OF 

 PHENACOMYS. 



From left to right these belong to the following species : No. 1, Phen- 

 acomys (Phenacomys) intermedius; No. 2, Phenacomys (Phenacomys) 

 orophilus; No. 3, Phenacomys (Arborimus) albipes; No. 4, Phenacomys 

 (Arborimus) longicaiidns. Traced from photograph. A little less than 

 six times natural size. 



Note the development of the second outer triangle and the general dif- 

 ference in outline posteriorly in the new subgenus Arborimus. 



There is some variation in the tightness of the closure of 

 the second outer triangle in longicaudiis. In the young the 

 triangle is scarcely formed. In its place is a channel, opening 

 broadly both forward and back and showing on its external 



