There are two subspecies of milksnakes found within the range of this work. The range for the mexican milksnake (annulata)is considered to be west through Val Verde County. From Sanderson west it is considered to be the range of the New Mexico milksnake (celaenops.) I am not an academic technician, so I will not disputed this. However, I have found celaenops in Sanderson, the Davis Mountains and the Big Bend which look to me like annulata. I have also found annulata in Val Verde county which have characteristics for celaenops. For identification, I go strictly by range.
A N.M. milksnake from the river road, Brewster county.
This N.M. milksnake is from Musquiz Canyon in southern Jeff Davis County.
This celaenops is from Black Gap Road, southeastern Brewster County.
Although it is from a bit out of the Trans-Pecos, I am including this picture of a NM milksnake from Andrews County to show more variations.