124 



CALIFORNIA ACADEMY OF SCIENCES 



[Proc. 4th Ser. 



inner triangle is not tightly closed, nor is it in oropJiilns No. 

 95080. 



There are no characters in this tooth which distinguish 

 the subgenus Arborimus from certain members of the subgenus 

 Phenacomys. In Merriam's original description of the genus 

 he uses the following language with reference to the front 

 lower molar (1889, p. 31) : "First lower molar with a pos- 

 terior transverse loop, four greatly elongated internal triangles 

 or digitations, of which at least two are completely closed, an 

 anterior loop of variable shape, and three short external tri- 

 angles, of which at least one is completely closed." In the 

 illustration of the enamel pattern of celatus in Merriam's plate 



4 



Fig. 2— ENAMEL PATTERN OF MANDIBULAR TEETH OF 

 PHENACOMYS. 



No. \, Phenacomys (Phenacomys) intermedins; No. 2, Phenacomys 

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

 Phenacomys (Arborimus) longicaudus. Traced from photograph. About 

 six and one-fourth times natural size. 



Note that in Arborimus the antero-external loop of Mo is not closed 

 and tends to be smaller than in Phenacomys ; note the peculiar arrange- 

 ment of the second outer triangle in longicaudus ; and finally note the 

 comparative simplicity of Mg in Arborimus. 



