Photo by Bill Reynolds

Plains garter snake (Thamnophis radix) is a medium-sized, nonvenomous snake. It is found through Minnesota except in the arrowhead region, but is most common in the western and metro counties. It is similar in appearance to, and difficult to distinguish from, the much more common and widespread eastern garter snake (Thamnophis sirtalis sirtalis).

The two species are distinguished by the position of pale lateral stripes and by markings on the upper jaw. Plains garter snake has pale lateral stripes on the third and fourth scale rows counting up from the belly scales. Each of the scales on the upper jaw (labial scales) has a bold black outline. Eastern garter snake has pale lateral stripes on the second and third scale rows, and none or at most one or two of the labial scales have a bold black border.

http://minnesotaseasons.com/Reptiles/plains_garter_snake.html