_Discussion._--The two populations are recognized as subspecies because: (1) there is close resemblance, (2) the diagnostic characters pertaining to pattern are few and superficial, and (3) the geographic ranges are allopatric, but juxtaposed. It is probable that _muticus_ and _calvatus_ would be capable of interbreeding if they were not spatially isolated. It should be pointed out, however, that there is no evidence of intergradation between _muticus_ and _calvatus_ in the lower Mississippi Valley as has been reported for the subspecies of _T. spinifer_ (Conant and Goin, 1948), and that the degree of difference between _calvatus_ and _muticus_ is greater than that between some subspecies of _T. spinifer_. _Specimens examined._--All the localities listed below are plotted on the distribution map (Fig. 1). Only those specimens of _T. muticus muticus_ are listed that serve to delimit the range of _T. m. calvatus_. Fortunately, the identification of the specimens of _muticus_ is certain as all show the characteristic juvenile pattern, except the large female, TU 7543, from southeastern Louisiana. USNM 95133-34 (carapaces and plastrons only) and TU 17236 are females, which lack the diagnostic spotted pattern of _calvatus_; the former are referred to this subspecies on geographic grounds (Pearl River at Columbia, Mississippi). TU 17236, from the Amite River, is dubiously relegated to _calvatus_ on the supposition that this river and others in the Lake Ponchartrain drainage will yield the characteristic juveniles. _Trionyx m. calvatus_ (33 specimens): TU 13473, 16682, 17301, 17302-.1, KU 47116 (skull only), Escambia River, 2 miles east, 1 mile north Century, Escambia Co., Florida; TU 17303-.4, 17304-.3, Pearl River, Varnado, Washington Par., Louisiana; TU 17306-.3, Pearl River, 9 miles south Monticello, Lawrence Co., Mississippi; TU 16956, KU 47117-19, USNM 7655, Pearl River, vicinity of Monticello, Lawrence Co., Mississippi; TU 17236?, Amite River, near Baton Rouge, Louisiana; TU 13795, Bogue Chitto River, Enon, Washington Par., Louisiana; TU 17305, no data, Louisiana; USNM 95133-34, Pearl River, Columbia, Marion Co., Mississippi; UI 31071, Pearl River, 14 miles northeast Jackson, Rankin Co., Mississippi; Uncatalogued, see page 523, Leaf River, 3 miles southeast New Augusta, Perry Co., Mississippi. _Trionyx m. muticus_ (6 specimens): TU 5989, Ouachita River, Monroe, Ouachita Par., Louisiana; TU 7543, Vacherie, St. James Par., Louisiana; CNHM 7845, Gayles, Caddo Par., Louisiana; USNM 92605, Greenville, Washington Co., Mississippi; USNM 113228, Jonesville, Catahoula Par., Louisiana; USNM 118167, Wheeler Reservoir, Tennessee River, Alabama.