species
may also be a derivative of the mentioned stock of the lower Rio
Grande.

The proposed former isolation of the basin of Cuatro Cienegas is
supported by evidence found in studies of other turtles in the basin.
Of the four kinds of turtles known to occur there (_Terrapene
coahuila_, _P. s. taylori_, _Trionyx spinifer emoryi_, and _Trionyx
ater_), all but _T. spinifer_ seem to be endemic. These three kinds
comprise a graded series, in regard to their degree of differentiation
from closest known relatives, as follows: 1) _Terrapene coahuila_ is
morphologically the most generalized and primitive of living box
turtles; the species is unique in its highly aquatic mode of life (see
Legler, 1960:532-534, for brief discussion of relationships within
genus Terrapene); 2) _Trionyx ater_ seems to represent a relict
population of pre-_Trionyx spinifer_ stock; presumably, _spinifer_ has
reinvaded the basin of Cuatro Cienegas in relatively recent times
and, as noted above, _spinifer_ and _ater_ now occur sympatrically (at
least in a geographic sense) in the basin (Webb and Legler, _op.
cit._); and, 3) evidence presented above suggests that _P. s. taylori_
intergrades with _P. s. elegans_ outside the basin.

The three endemic populations of turtles at Cuatro Cienegas therefore,
differ by varying degrees from their closest living relatives. This
variation in degree of difference possibly results from varying
periods of isolation. Probably the basin of Cuatro Cienegas has been
isolated from, and reconnected to, the lower Rio Grande drainage at
several times in the past. The relationships of fishes in the basin,
now under study by other workers, also suggest that the basin was
isolated more than once.

_Remarks._--Local names for the above-mentioned localities in the
basin of Cuatro Cienegas are as follows: Anteojo (6 mi. W Cuatro
Cienegas); El Mojarral (8.5 mi. SW); and Ojo de Agua de Tio Candido,
on Rancho Orozco (16 km. S). The Rio Chiquito is referred to by some
natives as "Rio Colorado" and by some as "Rio Salado." The local name
for _P. s. taylori_ is _tortuga negra_ (the name is used also for
_Terrapene coahuila_).

_Acknowledgments._--For permission to examine specimens in their care,
I wish to thank Doris M. Cochran, Smithsonian Institution (USNM),
Ernest E. Williams, Museum of Comparative Zoology (MCZ), Rollin H.
Baker, Michigan State University (MSU), Hobart M. Smith, University of
Illinois (IU), and Robert F. Inge