Fig. 4. Diagrammatic drawing of stretched skin of T. s. ornata. The ground color is like that of parietalis but paler with a continuous black area bordering the dorsal stripe (× 2-1/2).
T. s. parietalis differs from fitchi in several trenchant characters, and there are additional slight or average differences between the two. In approximate order of their importance the differences are as follows: 1) The red (or pale yellow or green or buffy) marks on skin between the scales on the upper half of the dorsolateral area (that is between the sixth and seventh, seventh and eighth and eighth and ninth scale rows) present in parietalis are missing in fitchi or are represented by only an occasional small fleck. 2) The dorsolateral area is black or nearly so in fitchi but averages paler in parietalis, in which a wide range of shades may be found from black to olive brown. 3) The red of the dorsolateral area frequently invades the lateral stripe, which sometimes is mostly red, and may even invade the ventrals in parietalis, but in fitchi the red marks are usually confined to the dorsolateral area, and do not invade the lateral stripe. 4) The prominent paired black dots or semicircular marks on the anterior edge of each ventral in parietalis are largely lacking in fitchi, which rarely has any dark marks on the ventral surface. 5) The dorsal stripe consistently involves the middorsal scale row and the adjacent half of the next row on each side, and is bright yellow in fitchi, but in parietalis it may be slightly wider, may be duller with more dusky suffusion, and its edges may be less sharply defined.
Intermediate and Atypical Populations
Of many specimens examined from eastern Oregon, Idaho, Utah, Wyoming and Colorado, few were typical of either parietalis or fitchi. Many were intermediate in some respects or showed a composite of characters of the two subspecies. No well-defined belt of intergradation exists, but the transition extends over more than a thousand miles, with local populations somewhat isolated and slightly differentiated along divergent lines. In view of this situation some plausibility could be claimed for any of several dividing lines between the subspecies. However, by far the most logical division is the Continental Divide; south of the Teton Range it constitutes a broad barrier separating eastern and western populations. Across Montana and Canada also it constitutes a more or less formidable barrier, with high altitudes and cold climates that probably are limiting to garter snakes. With few exceptions the snakes from east of the Continental Divide are more nearly like parietalis in the sum of their characters whereas those from west of the Divide are more nearly like fitchi.
In the Teton Range and in Yellowstone National Park these garter snakes occur in headwater streams up to the Continental Divide. KU 27956 from Two Ocean Lake 3-1/2 miles northeast of Moran, Teton County, Wyoming, agrees in its characters with fitchi, having the red lateral marks small and inconspicuous, discernible only on the anterior half of the body. The dorsolateral area is dark, almost black. The ventrals lack dark markings.
In Utah, populations of sirtalis occur in the drainages of the Bear, Weber and Sevier rivers and other smaller streams of the western half of the state. Obviously the species invaded Utah from the north, probably at a time when Lake Bonneville, the predecessor of the present Great Salt Lake, drained into the Snake River of Idaho. [Van Denburgh and Slevin] (1918:190) separated from their western "concinnus" and "infernalis" and allocated to parietalis the populations of Utah and southeastern Idaho, but presumably these authors were not familiar with typical parietalis of the Mid-west. Subsequent authors ([Wright and Wright], 1957:834; [Stebbins], 1954:505; [Conant], 1958:328) have followed this arrangement. A re-examination of specimens from Utah, including living individuals collected at Bear Lake in the summer of 1959, indicates that they should be assigned to fitchi rather than to parietalis.
Likewise various specimens from the drainage basin of the Snake River in Idaho are predominantly fitchi in the sum of their characters, although they differ from that subspecies in its most typical form and resemble parietalis in some respects. KU 23133 from two miles east of Notus, Canyon County, Idaho, has the red crescents on the lower part of the sides (between scale rows six and seven) consistently developed on the anterior half of the body. KU 21873, a large female from Bannock County, Idaho, is exceptional in having small lateral black spots on the ventrals, resembling parietalis most closely in this respect. Also, it has the red lateral crescents unusually well developed; the first three series are conspicuous, those of the fourth series are consistently developed, and those of the fifth series show occasionally.