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Universifij of California PuMications in Zoology [Vol. 21 



been designated in this paper as accessory cusps. The diagrams of 

 the second upper molars and fourth lower premolars indicate the 

 possible methods of the addition of these secondary cusps. 



A careful study of the enamel patterns of the molariform series 

 in each of nearly two thousand specimens, belonging to three species 

 of the genus Microtus, has conclusively shown that the closure or 

 opening of any particular section enclosed by enamel is extremely 

 uncertain. The amount of variation is so great that little reliance 

 can be placed on the use of any especial feature of this sort as a 

 diagnostic character. 



t F G H I J 



Figs. E-J. Diagrams of enamel pattern of fourth lower premolars of Microtus 

 montanus montanus, from Sisson, Siskiyou County, California. X 6. Fig. E, no. 

 2035, $, coll. A. B. Howell; fig. F, no. 2033, c^, coll. A. B. Howell; fig. G, no. 2038, 

 S, coll. A. B. Howell; fig. H, no. 2032, ^, coll. A. B. How^ell; fig. I, no. 98689, ^, 

 coll. Biol. Surv.; fig. J, no. 98r)88, $, coll. Biol. Surv. 



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

 mo)itanus montanus, from Sisson, Siskiyou County, California. X 6. Fig. K, no. 

 2028, (S, coll. A. B. Howell; fig. L, no. 2032, c^, coll. A. B. Howell; fig. M, no. 2003, 

 $, coll. A. B. Howell; fig. N, no. 2004, c?j eoU. A. B. Howell; fig. O, no. 21098, ^, 

 Mus. Vert. Zool.; fig. P, no. 2037, ^, coll. A. B. Howell. 



The exact enamel pattern of any particular second upper molar is 

 rarely if ever duplicated. The enamel patterns align themselves, 

 however, in certain categories, that is, modifications of the normal 

 enamel pattern of the tooth as is illustrated by the diagrams. It was 

 observed in the course of this study that nearly all degrees of varia- 

 tion in the enamel pattern were present to form a continuous series 



