The distinguishing characteristics for S. r. ravus include parietal scales that are highly variable in shape, less than 3 prefoveals, 21 midbody dorsal scales, 2-4 tail bands and a relatively large rattle.
Campbell and Lamar (2004) describe this species as being found across the Mexican Plateau in the temperate regions of moderate to high elevations. They estimate the vertical distribution to be from about 1,490 m above sea level to a little over 3,000 m altitude.
|Subspecies||Taxon author||Common name||Geographic range|
|S. r. brunneus||Harris & Simmons, 1977||Oaxacan pigmy rattlesnake||Mexico in the highlands of Oaxaca.|
|S. r. exiguus||Campbell & Armstrong, 1979||Guerreran pigmy rattlesnake||Mexico in the Sierra Madre del Sur of central Guerrero.|
|S. r. ravus||(Cope, 1865)||Central Mexican pigmy rattlesnake||Mexico in the Altiplanicie Meridional, including the states of México, Morelos, Tlaxcala, Puebla and Veracruz.|