Also called the Dusky Baja California rattlesnake, this medium-sized species is found on the San Quentin Plain in Baja California del Norte. The habitat in the San Quentin Plain is very rocky with brush, and it's more mesic than the surrounding deserts. The Rosario rattlesnake likely consumes primarily small mammals as an adult and utilizes burrows for shelter and to hunt. This species gives birth to less than ten neonates.

Extremely little is known about this species due to its very restricted and largely inaccessible habitat.

It is considered very closely related to C. enyo enyo; and the two are sister subspecies to C. enyo cerralvensis (the later which some scientists argue should be its own species); with both subspecies intergrading in areas where they overlap.

Fun fact: The name "furvus" is latin for "swarthy" or "of dark complexion", a reference to this subspecies' darker or "dusky" coloration compared to other Baja California rattlesnakes.