1896.] NATURAL SCIENCES OF PHILADELPHIA. 381 



In the M. i the triangles of one side are acute angled ; and of the 

 other, obtuse-angled. The posterior triangle presents an angle pos- 

 teriorly as well as laterally. In the M. ^ the same characteristics 

 exist, with the addition that the anterior (terminal) triangle has its 

 acute column pinched together, but not so as to exclude the dentine. 

 In the M. - the entering angle (groove) of one side enters the tri- 

 angle of the other side opposite to it, so as to destroy its triangular 

 character. The second triangle of the same side is also reduced by 

 the deep inflection of the opposite groove. Opposite the apex of 

 the second groove, a rudiniental 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 ante- 

 rior (terminal) column is flattened. Excepting on the M. ^, all the 

 triangles are well isolated. 



Measurements. m. m. 



f longitudinal ; 7.5 



Diameters M. ^ sp. no. 1 ^ anteroposterior ; 3.3 



1^ transverse ; 2 



f longitudinal ; 6 



Diameters M. ^ sp. no. 2 \ anteroposterior ; 2.7 



i^ transverse : 2 



f longitudinal ; 5.5 



Diameters M. ^ sp. no. 2 \ anteroposterior ; 3 



(^transverse; 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. - they are so far perfect as to show that the 

 pulp cavity is not closed below as in Evotoviys. 



Miorotus diluvianus sp. nov. 



The numerous species of the genus Microtus 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 read- 

 ily determined by dental characters, and as these are in all the spe- 

 cies 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 excellent work of Blasius on the Mammalia of 

 Europe (1859), it is possible to determine the relation of the Amer- 



