﻿26 



NORTH AMERICAN FAUNA. 



[No. 12. 



The value of the striictuie of the bony palate as a taxonoinic charac- 

 ter was first pointed out by Cones, ^ who, however, considered it of rather 

 more importance than it really is. It was at first supposed that the 

 bony palate of all the members of the geiuis Microtus differed in a con- 

 stant way from those of Evoiomys. Mr. Oldfield Thomas has, however, 

 recently described a Jlicrotus (J/. cMnensls) 

 in which the palate structure of Uvotomys is ^^^p 

 almost exactly reproduced ; and on further 



Fig. 1. — Firstuppermolar 

 iu six specimens of 



Fig. 2.— Second up- 

 per molar in six 

 specimens of JlTicro- 

 t US pe n n si/lva niciis . 



Fig. 3. — Third upper molar in 

 eighteen specimens of Microtus 

 pemuylvanicus. 



study it appears that several well-marked types may be recognized 

 among the species of the genus. These forms of palate furnish char- 

 acters of considerable worth in defining many subgenera. In all, 

 several structures remain sufficiently constant to serve as convenient 

 landmarks. The anterior portion of the bony palate, or that formed 



exclusively by the premaxillaries and 

 maxillaries, has no special interest, as 

 it shows very trifling variations. All 

 the characters of importance are de- 

 rived from the part lying behind the 

 maxillo-palatine suture. This suture 

 in the typi- al palate, or that occurring 



Fig. 4. — First lower molar iu 

 eighteen specimens of Microtus 

 pennnylvanicits. 



Fig. 5. — Second lower 

 molar in four speci- 

 mens of Microtus %>enn- 

 sylranicus. 



Fig. 6.— Third lower 

 molar in four specimens 

 of Microtus %>cnni<ylvan- 

 icus. 



in true Microtus and in the great majority of species and subgenera 

 (fig. 7 A) forms a broad, U -shaped loop, the convexity of which is directed 

 forward and whose apex lies about opposite the middle of the second 

 molar. From this point the suture on each side sweeps rapidly back- 

 ward and outward until, at the level of the anterior edge of the posterior 

 molar, practically^ the whole width of the palate is occupied b}^ the pala- 

 tine, and the maxillaries are reduced to a narrow rim around the edges 

 of the alveoli. 



1 Monogr. X. Am. Rodeutia, p. 133, 1877. 



