Thamnophis sirtalis parietalis Say was described (as Coluber parietalis) in 1823 from a specimen obtained in what is now Washington County, Nebraska, on the west side of the Missouri River three miles upstream from the mouth of Boyer's River [Iowa], or approximately eight miles north of Omaha. Although the type locality was unequivocally stated in the original description, Nebraska was not mentioned since the state was not yet in existence. Because the mouth of Boyer's River, the landmark by means of which the type locality is defined, is in Iowa, the impression has been imparted that the type locality itself is in Iowa (Schmidt, 1953:175), and to our knowledge the type locality has never been associated with Nebraska in the literature.

Like all the more western subspecies, parietalis is strikingly different from typical sirtalis in having conspicuous red markings. The relationship between the two was early recognized. Several of the other subspecies were originally described as distinct species. Coluber infernalis Blainville, 1835; Tropidonotus concinnus Hallowell, 1852; Eutainia pickeringi Baird and Girard, 1853; and others now considered synonyms eventually came to be recognized as conspecific with Thamnophis sirtalis. Ruthven (1908:166-173) allocated all western sirtalis to either parietalis or concinnus, the latter including the populations of the northwest coast in Oregon, Washington and British Columbia.

Subsequent more detailed studies by later workers with more abundant material led to the recognition of some subspecies that Ruthven thought invalid and led to the resurrection of some names that he had placed in synonomy. Van Denburgh and Slevin (1918:198) recognized infernalis as the subspecies occurring over most of California and southern Oregon, differing from more northern populations in having more numerous ventrals and caudals and a paler ground color. Fitch (1941:575) revived the name pickeringii for a melanistic population of western Washington and southwestern British Columbia, restricting the name concinnus to a red-headed and melanistic population of northwestern Oregon, and restricting the name infernalis to a pale-colored population in the coastal strip of California.

These changes left most of the populations formerly included in concinnus and infernalis without a name, and Fitch (op. cit.) revived Thamnophis sirtalis tetrataenia (Cope) to apply to them. However, Fox (1951:257) demonstrated that the type of T. s. tetrataenia came from the San Francisco peninsula (rather than from "Pit River, California" as erroneously stated in the original description) and that the name was applicable to a localized peninsular population rather than to the wide-ranging far western subspecies, which he named T. s. fitchi. The range of fitchi includes California west of the Colorado and Mohave deserts (except for the narrow strip of coast occupied by infernalis and tetrataenia), Oregon except the northwestern part, Washington east of the Cascade Range, most of British Columbia, extreme southeastern Alaska (occurring farther north than any other terrestrial reptile of North America) and parts of Idaho.

Neither Fox (1951) nor Fitch (1941) defined the eastern limits of fitchi or discussed its relationship to the subspecies parietalis. Wright and Wright (1957:849) stated: "Fitch ... did not even mention the big scrap basket form parietalis, from which he pulled T. s. fitchi (old tetrataenia). That comparison remains to be made, and the east boundary of fitchi and the west boundary of parietalis are still nebulous." We have undertaken to define better than has been done before the ranges of parietalis and fitchi and to list the diagnostic characters separating these two subspecies. Freshly collected material of both has been compared. At the time of his 1941 revision the senior author had never seen a live or recently preserved specimen of parietalis.