_Acknowledgments._--A portion of this study was completed in the summer of 1958, while I was an internee at the United States National Museum. I am grateful to Dr. Doris M. Cochran of that institution for helpful information regarding the origin of the type and for permission to study other specimens in her care. Thanks are due also to Messrs. Pete S. Chrapliwy and Kenneth L. Williams who collected two of the paratypes and who generously provided their notes on the specimens when learning of my study, to Dr. Hobart M. Smith and Mr. Charles M. Bogert for offering helpful suggestions and for the loan of specimens, and to Mr. Jebb Taylor for assistance with taking data. The drawings are the work of Mrs. Connie Spitz; the photographs are by the author. LITERATURE CITED CARR, A. 1952. Handbook of turtles; the turtles of the United States, Canada, and Baja California. Cornell Univ. Press, xv + 542 pp., 37 figs., 82 pls., 15 tables, 23 maps. DUGES, A. 1888. La Tortuga Polifemo. La Naturaleza, 1(ser. 2):146-147. 1896. Reptiles y Batracios de los E. U. Mexicanos. La Naturaleza, 2(ser. 2):479-485. FENNEMAN, N. M. 1931. Physiography of western United States. McGraw-Hill, New York, 1st. ed., v-xii + 534 pp., 173 figs. MILLER, L. 1932. Notes on the desert tortoise (_Testudo agassizii_). Trans. San Diego Soc. Nat. Hist., 7:187-208. SMITH, H. M. 1949. Herpetogeny in Mexico and Guatemala. Ann. Assn. American Geographers, 34(3):219-238, 1 fig. THAYER, W. N. 1916. The physiography of Mexico. Journ. Geol., 24:61-94, 2 figs. WOODBURY, A. M., and HARDY, R. 1948. Studies of the desert tortoise, _Gopherus agassizii_. Ecol. Monogr., 18:145-200, 25 figs., 4 tables. _Transmitted November 18, 1958._