_T. ater_ probably is a small species resembling _T. muticus_ and _T. spinifer emoryi_. The head is wide in _T. ater_, resembling that of _T. ferox_, and closely approaching that of _T. spinifer emoryi_ and _T. s. guadalupensis_. _T. ater_ resembles _T. ferox_ and _T. s. emoryi_ in having a narrow carapace. _T. ater_ resembles _T. s. emoryi_, _T. s. guadalupensis_ and _T. s. pallidus_, but differs from _T. muticus_, _T. ferox_ and the other subspecies of _T. spinifer_ in having the carapace widest farther posterior than one-half the length of the carapace. _T. ater_ resembles _T. ferox_ and _T. s. emoryi_ in shortness of snout. The plastron is short in _T. ater_ and most closely resembles that of _T. s. pallidus_, _T. s. guadalupensis_, and _T. s. emoryi_. _Remarks._--_T. ater_ is confined to permanent, clear-water ponds in the basin of Cuatro Ciénegas. The male and 11 females (KU) were taken at the type locality (a pond known locally as Tío Candido); the other female (UI 43510) was taken from a pond approximately seven miles northward (known locally as Anteojo). _T. spinifer emoryi_ also occurs in the basin of Cuatro Ciénegas. Males and females of _emoryi_ were collected in the Río Mesquites (Río Salado drainage) that drains the basin; two adult males of _emoryi_ were taken from the clear-water ponds--one from the type locality of _ater_ (KU 46907), and the other (KU 53757) from a pond (known locally as El Mojarral) from which no _ater_ were obtained. This demonstrated sympatry indicates that the two kinds are not conspecific. However, the nature and frequency of occurrence of characters of _T. ater_, suggest that it is subspecifically related to _T. spinifer_--in effect, a darkened race of _T. s. emoryi_. The diagnostic characters of fine corrugations on the posterior margin of the carapace and blackish marks on the ventral surface do not occur on every female of _ater_. Too, the dorsal coloration of living females (dark brown-buff) is paler than that of preserved specimens (dark gray-slate). Furthermore, a hatchling (CNHM 47367) recorded from Cuatro Ciénegas, Anteojo, is not distinguishable from _emoryi_.