﻿July, 1S9G.] 



SUBGENUS PITYMYS. 



59 



m 3 normally Tvith 2 or 3 closed triangles and. 6 salient angles. 



Mamm;p, 4. 



Plantar tubercles, 5. 



Sole moderately liairy. 



Claws on front feet longest. 



Fur short, dense, and mole like. 



SI'kU. — The skulls of tlie species of FHijmys differ considerably among 

 themselves. In Microtus xyinetorum (PI. I, fig. 2), the most highly modi- 

 fied, the brain case is very broad and flat and the interorbital region is 

 remarkably wide. The brain case is like that of Lagurus, bnt the broad 

 anterior part of the skull is very different from the latter. The dorsal 

 outline is strongly arched, especially anteriorly from the region between 

 the orbits to the tips of the nasals. The arching is, however, no more 

 strongly marked than in Microtns arvalis. In Microtus siihterranms 

 the skull is like that of M. x^lnetorum^ but the peculiarities are less 

 accentuated. In the Mexican species of Fiiymys the brain case is 

 narrower and higher than in J/, pinciorum^ and the anterior part of 

 the skull is less heavily built. The zj^gomatic 

 processes of the maxilla^ stand out more 

 nearly at right angles with the side of the 

 skull, thus bringing the broadest part of the 

 zygomatic arch farther forward than in 21, 

 innetoyum. 



Bony ijalaie. — The palate is normal, though 

 the region between the posterior molars is in ^ 

 M. plneiorum rather flatter than usual in true yig. si.— Enamel pattern of 

 Microtus, and the anterior outline of the inter- ^'^^^^ ^'?etii, («) 2iicrofus 

 pterygoul fossa IS otren somewhat hastate. jr. (P.) savu. (xs.) 



Enamel pattern in general. — With the excei)- 

 tion of the front lower molar and back upper molar, the enamel pattern 

 (fig. 31) is that of tetramerodont Microtus. 



Front lower molar. — The anterior mandibular tooth contains the same 

 number of loops and angles as the corresponding tooth in Microtus 

 arvalis. As a rule, however, the first and second triangles are not 

 completely isolated from each other or from the anterior looi). The 

 tooth is therefore exactly as in Fedomys. 



BacJc upjjer molar. — The posterior maxillary tooth is simplest in the 

 American species of the subgenus. In these it is like the back upper 

 tooth in Fedomys and Arvieola, which contain two closed triangles 

 and an anterior and posterior loop. In M. subterraneus, however^ the 

 tooth is formed exactly as in M. arvalis, while in M. seivii it is some- 

 what intermediate. In the last-named species the terminal loop is 

 slightly larger than in M. pinetorum. and a third closed triangle is 

 usually cut oft' from the outer base. 



Other teeth. — There is nothing peculiar about the incisors or remain- 

 ing molars. 



Mamma\ — In Fitymys there are only four mammje — all inguinal. 



