Crotalus viridis (Common names: prairie rattlesnake, Great Plains rattlesnake,) is a venomous pit viper species native to the western United States, southwestern Canada, and northern Mexico. Currently, two subspecies are recognized, including the prairie rattlesnake (Crotalus viridis viridis), the nominate subspecies, and the Hopi rattlesnake (Crotalus viridis nuntius). Taxonomy The taxonomic history of this species is convoluted. Previously, seven other C. viridis subspecies were also recognized, including C. v. abyssus, C. v. caliginis, C. v. cerberus, C. v. concolor, C. v. helleri, C. v. lutosus and C. v. oreganus. However, in 2001 Ashton and de Queiroz described their analysis of the variation of mitochondrial DNA across the range of this species. Their results agreed broadly with those obtained by Pook et al. (2000). Two main clades were identified, east and west of the Rocky Mountains, which they argued were actually two different species: on the one hand C. viridis, including the conventional subspecies C. v. viridis and C. v. nuntius, and on the other C. oreganus, including all the other traditional subspecies of C. viridis. The authors retained the names of the traditional subspecies, but emphasized the need for more work to be done on the systematics of C. oreganus. Description This species commonly grows to more than in length. The maximum recorded size is . In Montana, specimens occasionally exceed in length; the species reaches its maximum size in this region. One of the most characteristic features is the presence of three or more, usually four, internasal scales. Identification characteristics will vary depending on which subspecies is encountered. Generally, prairie rattlesnakes are usually lightly colored in hues of brown. Patches of dark brown are often distributed in a dorsal pattern. A color band may be seen at the back of the eye. The prairie rattlesnake group carries the distinctive triangle-shaped head and pit sensory organs on either side of the head. A key characteristic that can help differentiate a prairie rattlesnake from other rattlesnakes is the presence of two internasals contacting the rostral. Distribution and habitat Prairie rattlesnakes are found in North America over much of the Great Plains, the eastern foothills and some intermontane valleys of the Rocky Mountains, from southwestern Canada through the United States to northern Mexico. In Canada, they occur in Alberta and Saskatchewan; in the US in eastern Idaho, most of Montana (where it is one of 10 snake species and the only venomous