The common garter snake (Thamnophis sirtalis) is a species of natricine snake, which is indigenous to North America and found widely across the continent. Most common garter snakes have a pattern of yellow stripes on a black, brown or green background, and their average total length (including tail) is about 55 cm (22 in), with a maximum total length of about 137 cm (54 in). The average body mass is 150 g (5. 3 oz). Common garter snakes are also the state reptile of Massachusetts.