iffer sharply as to the extent of its distribution
in Texas, New Mexico and Arizona, but all show its distribution as
continuous over the more northern Great Basin and Rocky Mountain
states. However, specimens and specific locality records from
this extensive area seem to be scarce and some are based on early
collections of doubtful provenance. Throughout this region the low
rainfall, fluctuating and uncertain water supply, and general lack of
mesic vegetation along many of the streams render the habitat rather
hostile to garter snakes in general. _Thamnophis elegans vagrans_,
highly adapted to conditions in this region and generally distributed
over it, doubtless offers intensive competition to the species
_sirtalis_ wherever they overlap and perhaps constitutes a limiting
factor for _sirtalis_ in some drainage basins.

Convincing records of _sirtalis_ are lacking from all of
Colorado--except for those in the drainage basins of the South Platte,
and the Rio Grande east of the Continental Divide--from the eastern
half of Utah (east of the Wasatch Range), from New Mexico except for
the Rio Grande drainage (with one record each for the Canadian and
Pecos river drainages), from southwestern Wyoming (at least that
part in the Colorado River drainage basin), from the western half of
Oklahoma, and from Texas, except the eastern and extreme western and
northern parts. The species occurs in Nevada only near that state's
western and northern boundaries. The range is therefore much different
than it has been depicted heretofore, with the populations living east
of the Continental Divide widely separated from those to the west
for the entire length of the Rocky Mountains south of the Yellowstone
National Park region. The populations of northern Utah, southern
Idaho, and Nevada, which have been considered _parietalis_ are thus
far removed from the main population of that subspecies to the east
and are isolated from them by the barrier of the Continental Divide
and arid regions farther west.

Although some of the records published for _Thamnophis sirtalis_ are
erroneous, being based on misidentifications of other species, various
outlying records, including those in western Kansas, the Panhandle of
Texas, and southeastern New Mexico probably represent localized relict
populations that have survived from a time when the species was more
generally distributed in this region. The population of _T. sirtalis_
in the Rio Grande drainage