1922] 



Kellogg: Synopsis of Microtus Mordax 



277 



hypsodont teeth of this subfamily are extremely variable and that 

 abnormal enamel patterns are likely to be found in any genus or 

 species. On the basis of those structures alone, classification would 

 be very difficult. It is also now certain that the variation in the 

 enamel folding within some species, particularly Microtus calif ormciis, 

 M. mordax, and M. monianus, is as great as the differences which were 

 formerly supposed to separate these species from one another. 



This variation is not necessarily geographical but may be present 

 in one colony, as was pointed out (Kellogg, 1922) in the case of certain 

 specimens from Sisson, California, of the Microtus montanus group. 

 This interesting fact would indicate that the elements in the enamel 

 folding are still in course of modification which in the future will 

 probabh' lead to distinct types. At present the mordax, montanus, 

 and californicus groups are structurally very much alike, though 

 they are widely different with respect to habits. 



pm, 



m, 



pm 



ml 



A • B 



Figs. A, B. Microtus mordax sierrae, diagi-ams of enamel pattern of left 

 lower and left upper niolarifonn series. X 6. No. 22409, ^, Mus. Vert. Zool. ; 

 Mono Meadow, Yosemite Park, California. 



Abbreviations: Pm4, fourth premolar; Ml, first molar; M2, second molar; 

 end, entoeonid; esd, entostylid; hy., hypocone; hyd, hypoconid; me., metacone; med, 

 metaconid; mesd, mesostylid; ml., metaconule; mts., metastyle; pa., paracone; pad, 

 paraconid; pi., protoeonule; pr., protocone; prd, protoconid; prcd, protoconulid ; 

 ps., parastyle; sec. ml., secondary metaconule; sec. pad, secondary paraconid; sec. 

 prcd, secondary protoconulid. 



The wear on teeth of the molariform series may be enhanced or 

 diminished in direct relation to the type of food habitually used. 

 Thus immature individuals or young adults are occasionally found 

 with worn teeth. Normallj^ in any rodent with hypsodont teeth, 

 growth and attrition are equal, but, when abnormal factors interfere, 



