382 PROCEEDINGS OF THE ACADEMY OF [1896. 



ican 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 

 North American species, and especially 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 con- 

 venience of definition of the larger groups. Thus, in the species of 

 the 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. There are al- 

 ways two on the external side ; but the posterior outer may be pro- 

 longed to the inner side, or this prolongation may be cut off into a 

 distant triangle. These divisions include the following species : 



A. Second inferior molar, triangles, | 1. 



1. Second superior molar, triangles, 1 §, Agricola Blasius. M. 



agrestis Europe. 



2. Second sup. molar triangles, 111, Myonomes Raf., M. riparius, 



E. N. Amer. ; M. principalis, N. W. N. Amer. 



3. Second sup. molar triangles, 1 f ; Morof »s Selys (=Hemiotomys 

 Seljs, Paludicola Bias., Tetramerodon Rhoads). 31. amp hi bins ; 

 M. nivalis; M.rattieeps; 31. campedris ; 3I.arvalis; 31.subterra- 

 neus; 31. savii, Europe; 31. xaniJiognathus ; 31. fownsendii; M. 

 arvicoloides, N. America ; 31. speothen ; 3f. sigmodus ; 31. invo- 

 lutus; 31. diluviami-s Extinct, N. Amer. 



A A. Second inferior molar, triangles, 1 t 1. 



4. Second super, molar, triangles, 1 f, Pitymys McMur. 31. pine- 

 iorum, N. Amer. ; 3J. didelta, Extinct, N. Amer. 



The large size of 3Iicrotus diluvianus Cope distinguishes it from 

 all the extinct and recent American members of the genus. It is 

 only represented by the M. i-^ of both sides, so that many of its 

 characters remain to be discovered. The triangle formula of these 

 teeth is M. i 1 I, M. ^ 1!. In both molars the triangles are acute 

 and are well closed, and the posterior one presents an angle poste- 

 riorly. The lateral keels are | and |. The valleys are wide open below. 



