Wherever it occurs at all, the common garter snake is usually abundant. Because of its diurnal habits and the concentration of its populations along watercourses, it is not likely to be overlooked. There are few, if any, remaining large areas in the United States where herpetologists have not carried on field work. It may be anticipated that certain rare and secretive species will still be found far from any known stations of occurrence, and seeming gaps in the ranges of these species will eventually be filled. But for the common garter snake the negative evidence provided by the lack of records from extensive areas should be taken into account in mapping the range.

Most large collections of garter snakes contain misidentified specimens. The diagnostic differences in color and pattern are often obscured, especially if the specimens are poorly preserved. Many specimens deviate from the scalation typical of the form they represent, and key out to other species. Isolated records should therefore be accepted with caution. A case in point is Colorado University Museum No. 46, from Buford, Rio Blanco County, Colorado, originally identified by Cockerell (1910:131) as Thamnophis sirtalis parietalis. This specimen, and another, now lost, from Meeker in the same county seemingly served as the basis for mapping the range of sirtalis across the western half of Colorado, for there seem to be no other records from this part of the state. However, a re-examination of the specimen from Buford shows it to be an atypical individual of another species, T. elegans vagrans. A specimen of T. radix haydeni (Col. U. Mus. No. 3165) was the basis for Maslin's (1959:53) record of parietalis in Baca County on the north fork of the Cimarron River in southeastern Colorado. Brown (1950:203) has mentioned the difficulty of defining the range of sirtalis in the southern Great Plains because of misidentifications of the similar T. radix.

The range of the common garter snake has never been adequately mapped in the Rocky Mountain and Great Basin states. Recent general works (Smith, 1956:291; Wright and Wright 1957:834; Stebbins 1954:505; Conant 1958:328) which have shown maps of the over-all range of sirtalis, differ 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 RÍo Grande east of the Continental Divide—from the eastern half of Utah (east of the Wasatch Range), from New Mexico except for the RÍo 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 RÍo Grande drainage 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.

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 their dorsal stripes were reddish orange, and markings on the dorsolateral area were pale yellow rather than red. Specimens from the Texas Panhandle, from Hemphill County (Brown, 1950:207) and nine miles east of Stinnet, Hutchison County (Fouquette and Lindsay, 1955:417) likewise are most nearly like annectens judging from the authors' descriptions. The specimens from nine miles east of Stinnet averaged large; the two largest would have attained or slightly exceeded four feet in length if they had had complete tails. No sirtalis so long as four feet has been recorded elsewhere.

Records are lacking from the drainages of the Republican, North Canadian, Brazos and Colorado River drainages in the High Plains, but possibly isolated populations occur in some of these also. The only record from the Pecos River drainage is that of Bundy (1951:314) from Wade's Swamp near Artesia, Eddy County, New Mexico. This locality is separated by some 140 miles from any other known station of occurrence.

From extreme southern Colorado south across New Mexico to the Mexican border T. sirtalis occurs in continuous or nearly continuous populations in the RÍo Grande Valley, and has been recorded from many localities. It has been recorded from relatively few localities of tributary streams (Los Pinos, Abiqui, Santa Fe) all near the main valley. There is one record from the Ocate River, a headwaters tributary of the Canadian River, in the Sangre de Cristo Mountains near other localities in the RÍo Grande drainage. The southwestern-most known locality of occurrence is Casas Grandes in the Mexican state of Chihuahua some 130 miles southwest of El Paso, Texas, and near the Continental Divide. The RÍo Casas Grandes must have once been a tributary of the RÍo Grande, but now its desert drainage basin is isolated.