Some skulls of soft-shelled turtles from streams of the Atlantic Coast drainage, including the skull of the holotype of _Platypeltis_ (= _Trionyx_) _agassizi_ Baur (MCZ 37172, Pl. 54), show at least two differences from other skulls of _asper_ and from those of other subspecies of _T. spinifer_. Figure 20 shows that skulls of _agassizi_ tend to have slightly smaller internal choanae (ratio IC/MB) than those of _T. spinifer_ and _T. ferox_; there is seemingly little difference between skulls of _ferox_ and _spinifer_, and little, if any, ontogenetic variation. Figure 21 shows that most skulls of the _agassizi_-form that exceed 43.0 millimeters have a more expanded, alveolar surface of the maxilla than skulls of _spinifer_ of approximately the same size; most skulls exceeding a basicranial length of 43.0 millimeters, and certainly all skulls exceeding 50.0 millimeters are those of females. Stejneger (1944:Pl. 30) also has provided photographs of a skull of the _agassizi_-form. It is of interest that of the 12 _agassizi_-form skulls (MCZ 37172; USNM 8708, 029034, 51981, 66859, 71681, 91282, 91310-11, 92521, 92583-84) that I examined some resemble _ferox_ (Neill, 1951:9) in having the alveolar surfaces of the jaws broadened, and the greatest width at the level of the quadratojugal (Table 3, Plate 54); also, the localities of all 12 skulls are within the geographic range of _ferox_. Skulls of _ferox_, however, have conspicuously broadened alveolar surfaces of the jaws only when they exceed in length the largest skulls of _agassizi_. The differences of skulls of the _agassizi_-form possibly reflect isolation in the Atlantic Coast drainage, and an adaptation in feeding habits. So far as I can ascertain, individuals occurring in rivers of the Atlantic Coast drainage in Georgia and South Carolina (referable to _agassizi_) do not differ consistently in external characters from individuals of _T. s. asper_ that occur westward in the Apalachicola drainage. _Comparisons._--_Trionyx s. asper_ can be distinguished from all other subspecies of _T. spinifer_ by usually having more than one black line paralleling the rear margin of the carapace. This character and the frequent fusion of the postlabial and postocular stripes on the side of the head distinguish _asper_ from _spinifer_ and _hartwegi_. _T. s. asper_ differs from _pallidus_, _guadalupensis_ and _emoryi_ in having blackish spots and ocelli on the carapace, and lacking whitish dots or tubercles. _T. s. asper_ resembles _spinifer_, _hartwegi_ and _pallidus_ but differs from _guadalupensis_ and _emoryi_ in having conical tubercles along the anterior edge of the carapace in large females. For additional differences see accounts of other subspecies.