_Acknowledgments._--For permission to examine specimens in their care, I wish to thank Doris M. Cochran, Smithsonian Institution (USNM), Ernest E. Williams, Museum of Comparative Zoology (MCZ), Rollin H. Baker, Michigan State University (MSU), Hobart M. Smith, University of Illinois (IU), and Robert F. Inger, Chicago Natural History Museum (CNHM). Pete S. Chrapliwy, John K. Greer, Robert G. Webb, and Kenneth L. Williams all contributed field data concerning the specimens of _P. s. taylori_ that they collected. I am especially grateful to Webb for donating two specimens to the University of Utah (UU). Special gratitude is expressed to Wendell L. Minckley and Robert B. Wimmer for assistance with field work at Cuatro Ciénegas. Daniel Rodríguez, Cuatro Ciénegas, guided us to the various ponds at and near the type locality. Robert R. Miller, Robert G. Webb, and Donald Tinkle read the manuscript and offered helpful criticisms. Figures 1 and 2 were drawn by Lorna Cordonnier. _Comparative materials examined_ (total of 135 specimens).--_P. s. elegans_ (52 specimens): KU 2897-8, 3195, 18337, 18341, 18345, 18347, 18364, 45027-31, 45033, 46750, 46863, and John M. Legler 1394 and 1435, various localities, Kansas; KU 16400, Howard Co., Texas; KU 39983-4, 8 mi. N and 2 mi. W Piedras Negras, Coahuila; KU 33525, 33527-9, La Gacha, Coahuila; CNHM 28843-5, 55625-45, Rancho las Ruscias, Múzquiz, Coahuila; KU 39982, 2 mi. S and 3 mi. E San Juan de Sabinas, Coahuila; KU 33524, Don Martín Reservoir, Coahuila; _P. s. elegans_ × _taylori_ (3): KU 53785-7, 8 mi. W. Nadadores, Coahuila; _P. s. gaigeae_ (39): MCZ 54724, Elephant Butte Reservoir [Sierra or Socorro Co.], New Mexico; KU 51158-61, 51202-3, Lajitas, Brewster Co., Texas; KU 51162-6, 51204-6, 51315, 1 mi. NW Ojinaga, Chihuahua; KU 33884, 51167-72, 51207-20, 3 mi. N and 5 mi. E Meoqui, Chihuahua; IU 43583-4, La Cruz, Chihuahua; _P. s. ornata_ (9): MCZ 46392-3, Río Purificación, Rancho Sta. Ana, Tamaulipas; MCZ 46397, E of Güémez, Tamaulipas; MCZ 46400, Jiménez, Tamaulipas; KU 40161-2, Alvarado, Veracruz; KU 40131, San Andrés Tuxtla, Veracruz; V. E. Thatcher 98, 15 mi. N Teapa, Tabasco; KU 40139, Cantemo[c], Tabasco; _P. s. taylori_ (23 in addition to type series): KU 51438, 51442, 53788-53801 topotypes; KU 53802-5, 8.5 mi. SW Cuatro Ciénegas, Coahuila; KU 51439-41, 10 km. S Cuatro Ciénegas, Coahuila; _P. floridana texana_ (10 from Coahuila): KU 39985, 2 mi. W Jiménez; CNHM 55654, Allende; CNHM 55646, Cd. San Juan; CNHM 55648, Hermanas; CNHM 55649-53, Lampacitas; KU 33526, Don Martín Reservoir. LITERATURE CITED CARR, A. 1952. Handbook of turtles: the turtles of the United States, Canada, and Baja California. Cornell Univ. Press, xv+542 pp., 82 pls., 37 figs., 15 tables, 23 maps.