of New Mexico is geographically isolated
and remote from other populations of the species. Except for a few
isolated and highly localized populations the species is absent from
the Republican, Smoky Hill, Arkansas, Cimarron, Canadian, Red, Brazos,
Colorado and Pecos rivers and their tributaries west of the one
hundredth meridian in the arid High Plains.

Streams in this region of High Plains are in most instances unsuitable
habitats because they are in eroded channels, have a variable
and uncertain water supply, and have poorly developed riparian
communities. The marsh and wet meadow habitat preferred by _sirtalis_
in most parts of its range is almost absent. _T. radix_ and _T.
marcianus_, well adapted to conditions in this region, perhaps provide
competition that is limiting to _T. sirtalis_. However, several
well-isolated populations of _sirtalis_ have survived as relicts in
the southern Great Plains, presumably from a time several thousand
years ago when mesic conditions were more prevalent, perhaps in an
early postglacial stage.

* * * * *

Illustration: FIG. 1. Map of a part of the United States in
the region of the Great Plains and Rocky Mountains, and adjacent
northwestern Mexico showing supposed range (shaded) and localities of
authenticated occurrence (dots) of _Thamnophis sirtalis_. 1. _T.
s. fitchi_, 2. _T. s. parietalis_, 3. _T. s. annectens_, 4. _T. s.
ornata_. Records from Idaho and Wyoming are based on specimens in
the University of Kansas Museum of Natural History collection. Other
records are based on Woodbury (1931) for Utah, Hudson (1942) for
Nebraska, Maslin (1959) for Colorado, Smith (1956) for Kansas, R. G.
Webb (MS) for Oklahoma, Brown (1950) and Fouquette and Lindsay (1955)
for Texas, Cope (1900), Van Denburgh (1924), Little and Keller (1937)
for New Mexico, and Smith and Taylor (1945) for Mexico.

* * * * *

Smith (1956:292) recorded _parietalis_ from three outlying stations
in the western quarter of Kansas, from Wallace, Hamilton and Meade
counties in the drainages of the Smoky Hill River, Arkansas River, and
Cimarron River, respectively. Permanent springs in Meade County State
Park perhaps account for the survival of an isolated colony there.
Several specimens from that locality seen by Fitch in August, 1960,
when recently collected by a University of Michigan field party,
seemed to be of the Texas subspecies _annectens_, as