. Bulletin. Natural history; Natural history. 232 Illinois Natural History Survey Bulletin Vol. 28, Art. 1. Fig. 218.—An adult Thamnophis sirtalis sirtalis from Douglas County, Illinois. The groundcolor is brown on which are superimposed black markings and three longitudinal tan stripes. Table 69.—Sexual variation in Illinois Thamnophis sirtalis. Figures in parentheses are numbers of specimens. Males (38) Females (54) Characteristic Range Mean Range Mean Ventrals 150-163 70-82 147-165 61-74 Caudals Tail length as percentage of total length al
. Bulletin. Natural history; Natural history. 232 Illinois Natural History Survey Bulletin Vol. 28, Art. 1. Fig. 218.—An adult Thamnophis sirtalis sirtalis from Douglas County, Illinois. The groundcolor is brown on which are superimposed black markings and three longitudinal tan stripes. Table 69.—Sexual variation in Illinois Thamnophis sirtalis. Figures in parentheses are numbers of specimens. Males (38) Females (54) Characteristic Range Mean Range Mean Ventrals 150-163 70-82 147-165 61-74 Caudals Tail length as percentage of total length alleged diagnostic character of parietalis, appears occasionally in populations of T. sir- talis throughout Illinois. Thirty-three per cent of a sample of 12 snakes from north- western Illinois, 42 per cent of a sample of 20 snakes from east-central Illinois, 55 per cent of a sample of 20 snakes from west- central Illinois, and 26 per cent of a sample of 20 snakes from extreme southern Illinois have some red on the sides of the body. In view of the homogeneity of garter snakes in Illinois in scutellation and in color, there is no basis for including T. s. parie- talis as a part of the Illinois fauna, the nu- merous reports of it and of sirtalis X parie- talis intergrades in the western part of the state notwithstanding. In pattern, however, there is pronounced geographic variation among garter snakes- within Illinois. Toward the northeast the lateral stripes on the anterior part of the body tend to be interrupted at regular in- tervals by crossbars formed by the fusion of the black spots just above and just below the lateral line. The crossbarred pattern is sufficiently constant in garter snakes found along the southwestern border of Lake Michigan for the population to be recog- nized subspecifically as semifasciata. Inter- gradation between sirtalis and semifasciata is manifested by the increasing frequencies, toward the northeast, of specimens with the crossbarred pattern.
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