On the basis of morphological criteria, I suspect that _ater_ and _emoryi_ are genetically compatible. Possibly there is only sporadic entrance of _emoryi_ into the ponds inhabited by _ater_, or the accessible dispersal routes for _emoryi_ have been relatively recent and there has been insufficient time for genetic adaptation. _T. ater_ is maintained as a full species because of the occurrence of two distinct males (KU 46907, _emoryi_, and KU 46911, _ater_) in the same pond (Tío Candido, the type locality). These two specimens are contrasted in a photograph accompanying the type description (Webb and Legler, 1960: Pl. II). The restricted distribution of _ater_, and its characteristics suggest a relict population derived from a _ferox_-like ancestor that may be in the process of becoming extinct. There are two specimens in the CNHM recorded from Cuatro Ciénegas. One is a female (CNHM 55661) having a plastral length of 19.0 centimeters, and no specific locality other than Cuatro Ciénegas. I examined this specimen before I knew of the existence of _ater_, and noted no unusual features; I have not re-examined the specimen. It is considered representative of _emoryi_. The second is a hatchling (CNHM 47367) having a plastral length of 3.2 centimeters, recorded from Cuatro Ciénegas, Anteojo. The carapace is dark tan having small whitish dots intermixed with a few indistinct, small, blackish specks posteriorly. The specimen is indistinguishable from _emoryi_. _Specimens examined._--Total 12, as follows: COAHUILA: KU 46903-06, 46908-12, 53755-56, 16 km. S Cuatro Ciénegas; UI 73510, 5.7 mi. W Cuatro Ciénegas. _Records in the literature._--Schmidt and Owens (1944:103) record _emoryi_ from Cuatro Ciénegas (no museum numbers listed); presumably their reference is to CNHM 55661. =Trionyx muticus= Lesueur Smooth Softshell _Range._--United States from extreme western Pennsylvania, southern Minnesota and South Dakota south to the Gulf of Mexico in Alabama, the western end of the panhandle of Florida, and the eastern half of Texas (see map, Fig. 22.) [Illustration: FIG. 22. Geographic distribution of _Trionyx ater_ and _Trionyx muticus_. 1. _T. muticus muticus_. 2. _T. muticus calvatus_. 3. _T. ater_.] _Diagnosis._--Septal ridges absent; anterior edge of carapace smooth, lacking prominences; juvenal pattern of large dusky spots (sometimes ocellate), or small dusky (not black), dots and short lines; side of head usually devoid of markings except for pale, usually uninterrupted, postocular stripe.