1922] Kellogg: Calif ornian Forms of Microtus Montanus 



249 



TABLE II 



Table showing allocation of the skulls according to the different cranial elements selected for 

 measurement; all specimens from Sisson, Siskiyou County, Calitornia. 



DENTITION 



The writer's attention was first directed to the wide variation in 

 the enamel pattern of the niolariform series in the genus Microtus by 

 :\Iiller's (1896, p. 26) figures of the variation in enamel folding in 

 Microtus pennsylvoinicus. Miller estimates that 75 per cent of the 

 specimens possess a normal enamel pattern, though he illustrates with 

 diagrams eighteen types of variation in both the M^ and Pm^ which 

 were present in one hundred and seventy specimens of that species. 

 This variation is considerably greater than has been currently 

 assumed, and will be discussed somewhat fully. 



The author is indebted to Mr. Gerrit S. Miller, Jr., for advice as 

 to the nomenclature to be employed for the enamel crown divisions 

 of Microtus, but the writer himself is to be held responsible for any 

 errors he may have made in interpreting Mr. Miller's remarks as to 

 the homologies of the elements of the enamel pattern. Numerous 

 attempts have been made in the past to assign certain symbols or 

 names to the divisions of the molar cromis of rodents, but there has 

 been much disagreement as to the homologies of the divisions con- 

 cerned. Among the first to adduce specific evidence in support of 

 their findings were Winge (1883, pp. 67, 68-69) and Forsyth-Major 



