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.
The mention of absence of septal ridges in males of T. ater in the original description (Webb and Legler, 1960:22) should be amended. The septal ridges in the only known adult male are reduced; a small, whitish ridge is present on the medial surface of each nostril, but is not conspicuous in anterior view. The one adult male of ater is distinguished from T. s. emoryi principally on the over-all dark, dorsal coloration with concomitant loss of pattern, the noticeably broadened snout, and the reduced septal ridges. The last character mentioned possibly is variable in ater (and in emoryi in this region) in view of the variation in development of the ridge on four male emoryi from the basin: well-developed on KU 53757 (Mojarral) and KU 46907 (Tío Candido); reduced on KU 53752 (Río Mesquites), resembling development in ater; and, reduced on right side only on KU 53753 (Río Mesquites).
Presumably, the continued erosive action at the headwaters of the Río Salado has permitted the invasion of this drainage into the formerly isolated basin of Cuatro Ciénegas. In the basin, however, I know of no evidence of a direct aquatic contact between the headwater streams and the isolated, clear-water, ponds. How emoryi entered the ponds is unknown. Some of the ponds are tapped by small, man-made, irrigation canals, but, so far as I know, these are not connected to the river. The ponds have permanent water and are often separated by several miles of arid environment. Overland dispersal between waterways is possible in time of flooding. Local residents tell of the infrequent sale of softshells in Cuatro Ciénegas, which hints at their dispersal via the agency of man. The underlying gypsum substrate of the valley has been subjected to considerable erosion; the ponds observed have deep holes, and small caverns and grottos. There are conflicting reports concerning subterranean connections between ponds. Possibly there are underwater connections between some ponds and the headwater streams of the Río Mesquites. Whatever the dispersal route for emoryi into the ponds has been, it is strange that the same route has not been traversed by ater, permitting its occurrence in the Río Mesquites.
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.