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Universitif of California Puhlicatious in Zoology [Vol. 21 



other modifications are brought about in the skull by the action of 

 muscles controlling movements of the jaw, due in part to the unusual 

 stresses or strains arising from the impaired functioning of the grind- 

 ing teeth. Then, too, normally with advancing age the development 

 of ridges and tuberosities becomes more pronounced while the outlines 

 of certain bones change, the angularity of the skull increases, and 

 sutures tend to become obliterated by fusion of the adjacent bones. It 

 was therefore thought best to figure not only the skulls of typical 

 mature individuals, but the skulls of old adults as well. In each case 

 the second specimen to be figured of any given subspecies is the older 

 individual, so far as can be ascertained from a study of the skull. 



Figs. C-E. Diagrams of enamel pattern of second upper molars of Microtus 

 mordax mordax. X 6. Fig. C, no. 11192, ^, Mus. Vert. Zool., Warren Peak, War- 

 ner Mountains; fig. D, no. G7983, 5, Mus. Vert. Zool., Goose Lake, Modoc County; 

 fig. E, no. 11194, 5, Mus. Vert. Zool., Warren Peak, Warner Mountains. 



Figs. F-P. Diagrams of enamel pattern of second upper molars of Microtus 

 mordax sierrae. X 6. Fig. F, no. 95U67, ^, Biol. Surv. coll., Lassen Peak, Shasta 

 County; fig. G, no. 106169, $, Biol. Surv. coll.. Grizzly Mountains, Plumas County; 

 fig. H, no. 17448, ?, Mus. Vert. Zool., Little Onion Valley, Sierra Nevada, Inyo 

 County; fig. I, no. 23160, (^, Mus. Vert. Zool., Fletcher Creek, Yosemite Park, 

 Tuolumne County; fig. J, no. 17457, $, Mus. Vert. Zool., Little Onion Valley, 

 Sierra Nevada, Inyo County; fig. K, no. 95665, <^, Biol. Sui-v. coll., Lassen Peak, 

 Shasta County; fig. L, no. 15750, $, Mus. Vert. Zool., Little Biiish Meadow, 

 Olancha Peak, Tulare County; fig. M, no. 25024, S, Mus. Vert. Zool., Bullfrog 

 Lake, Fresno County; fig. N, no. 25035, $, Mus. Vert. Zool., Kings River Caiion, 

 Fresno County; fig. O, no. 25031, $, Mus. Vert. Zool.. Kings River Canon, Fresno 

 County; fig. P, no^ 13455, ,^, Mus. Vert Zool., Head Rush Creek, Siskiyou County. 



