Fossil material was not studied in detail. The fossil softshells indicate a more widespread, former distribution. Some osteological characters and their variation in the living species are mentioned as an aid to future workers concerned with an assay of fossil remains. Fossils occur in marine, brackish and fresh-water deposits, and many are much larger than the living species; the oldest American fossils are of Upper Cretaceous age. The interrelationships of the living species and subspecies suggest that the species _spinifer_, _ater_, and _muticus_ are derivatives of a _ferox_-like ancestor, and that they differentiated in North America; most differentiation occurs in southwestern Texas and northern México where characters of some populations indicate alliance with _ferox_. It is hypothesized that aridity in the late Tertiary effected specific differentiation by the modification and isolation of aquatic habitats. Pluvial periods in the Pleistocene provided for confluence of aquatic habitats and expansion of geographic ranges, and coupled with physiographic changes, conceivably caused or enhanced some of the subspecific variation. LITERATURE CITED References marked with an asterisk were not seen by the author. ADAMS, M. S., and CLARK, H. F. 1958. A herpetofaunal survey of Long Point, Ontario, Canada. Herpetologica, 14(1):8-10, April 25. ADLER, K. K., and DENNIS, D. M. 1960. New herpetological records from Ohio. Jour. Ohio Herp. Soc., 2(4):25-27. AGASSIZ, L. 1857. Contributions to the natural history of the United States of America. Vol. I. Part II. North American Testudinata. Vol. II. Part III. Embryology of the turtle. Little, Brown and Co., Boston, pp. 233-452d; pp. 449-643, 27 pls. ALLEN, E. R., and NEILL, W. T. 1950. The alligator snapping turtle _Macrochelys temminckii_ in Florida. Spec. Publ. Ross Allen's Rept. Inst., 4:1-15, 6 figs., May. ALLEN, W. B., JR. 1955. Some notes on reptiles. Herpetologica, 11(Pt. 3):228, November 30. ANDERSON, P. 1942. Amphibians and reptiles of Jackson County, Missouri. Bull. Chicago Acad. Sci., 6(11):203-220. ANDERSON, P. K. 1958. The photic responses and water-approach behavior of hatchling turtles. Copeia, 1958(3):211-215, 5 figs., August 28. ANTEVS, E. 1948. Climatic changes and pre-white man. _In_ The Great Basin with emphasis on glacial and postglacial times. Bull. Univ. Utah, 38(20), Biol. Ser., 10(7):168-191, 1 fig., 1 table, June 30. ATKINSON, D. A. 1901. The reptiles of Allegheny County, Pennsylvania. Ann. Carnegie Mus., 1901-1902, 1(4):145-157. BABCOCK, H. L. 1919. The turtles of New England. Mem. Boston Soc. Nat. Hist., 8(3): 325-431, 16 pls., April.