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ILLUSTRATIONS. 



13. Enamel pattern of Dicrostonyx from Ungava. , 



14. Ear of Dicrostonyx and Lemmas. 



15. Left front foot of tliree specimens of Dicrostonyx from Alaska, showing seasonal 



change in form of middle claws. 



16. Side view of molars of adult and young Phenaconiys. 



17. Enamel pattern of molar teeth of Phevacomys celatus. 



18. Side view of molars of adult and young Evotomys. 



19. Enamel pattern of molar teeth oi Evotomys gaj^peri. 



20. Side view of molars of adult Microtus. 



21. Left front foot of Microtus terrestris. 



22. Enamel pattern of molar teeth oi Microtus (Eothenomys) melanogaster. 



23. Enamel pattern of molar teeth of Microtus (Anteliomys) cliinensis. 



24. Audital bulhe of Microtus (Microtus) arvalis and 21. (Lagurus) palUdus. 



25. Enamel pattern of molar teeth of Microtus (Lagurus) luteus, M.(L.) lagurus, and 



M. (L.) pallidus. 



26. Enamel pattern of molar teeth of Microtus (Alticola) albicaiida. 



27. Audital bulhie of Microtus (Alticola) alhicauda and M. (Hyperacrius) fertilis. 



28. Enamel pattern of molar teeth of Microtus (Hyperacrius) fertilis. 



29. Enamel pattern of molar teeth of Microtus (Pedomys) austerus. 



30. Enamel pattern of molar teeth of Microtus (Phaiomys) strauchi, 



31. Enamel pattern of molar teeth of Microtus (Pitymys) pinetorum and M. (P.) savii. 



32. Enamel pattern of molar teeth of Microtus (Chilotus) oregoni. 



33. Enamel pattern of molar teeth of Microtus (Microtus) arvalis, M. (M.) nivalis, M. 



(M.) pennsylv aniens, and ikf, (M.) ratticeps. 



34. Enamel pattern of molar teeth of Microtus (Arvicola) terrestris and M. (A.) 



macropus. 



35. Enamel pattern of front lower molar of type of Microtus arvicoloides. 



36. Enamel pattern of molar teeth of Microtus (Neofiher) alleni. 



37. Dorsal view of skull of Fiber. 



38. Side view of molars of adult Fiber. 



39. Enamel pattern of molar teeth of Fiber zibethicus. 



40. Enamel pattern of molar teeth of 'Arvicola' intermedius. 



