204 VERTEBRATE REMAINS, PORT KENNEDY BONE DEPOSIT. 



second triangle of the same side is also reduced by the deep inflection of the oppo- 

 site groove. Opposite the apex of this second groove a rudi- 

 mental third triangle is present in the form of the section of 

 a keel of the surface. This I reckon as one of the three 

 divisions of the terminal lobe. The other two are not well 

 distinguished; one opposite to the keel just mentioned is an 

 acute angle, and the terminal one is strongly convex. Thus 

 on this tooth there are three keels on one side and four on 

 the other. The anterior (terminal) column is flattened. 

 Excepting on the m- 3 -, all the triangles are well isolated. 



Measurements. 



( longitudinal ; 

 Diameters m 1 sp. no. 1 < anteroposterior ; 



( transverse ; 



f longitudinal ; 

 Diameters m^ sp. no. 2 < anteroposterior ; 



( transverse ; 



f longitudinal ; 

 Diameters m- 3 - sp. no. 2 -l anteroposterior ; 



I transverse ; 



mm. 



7.5. 



3.3 



2 



6 



2.7 



2 



5.5 



3 



1.7 



The walls of the common pulp cavity are broken off in most of the teeth of 

 this species above described, but portions remain in most of them, and in the 

 m- 3 - they are so far perfect as to show that the pulp cavity is not closed below as in 

 Anaptogonia. 



MICROTUS Selys. 

 Etudes de Micromammalogie de l'Europe, 1839, p. 85. Arvicola Auctorum. 



The numerous species of this genus are distinguished into groups by various 

 characters, e.g., those of the molar teeth, of the size of the ears, tail, etc. The 

 extinct species can be most readily determined by dental characters, and as these 

 are in all the species less matters of proportion, and more a question of the number 

 of parts, they are to be preferred as possessing greater fixity. Thanks to the excel- 

 lent work of Blasius on the mammalia of Europe (1857) it is possible to determine 

 the relation of the American species to the types of the divisions proposed by 

 European authors. I am also much indebted to my friend Mr. S. N. Rhoads for 

 the opportunity of examining skulls of a number of rare N. American species, and 

 especially of those described by himself from the Pacific Coast. 



The species differ as to the number of triangles in the first inferior premolar. 

 There is, however, some lack of constancy in the relations of the anterior triangles 

 to the treffle, so that I have depended rather on the characters of the second molars 

 in both jaws for convenience of definition of the larger groups. Thus in the species 

 of M. pinetorum group, the last two triangles on one side fuse to a median position 

 similar to that of the first column. In the other groups, where this tooth has two 

 triangles on each side, the second superior molar differs in the number of its triangles. 



