It's a Mexican Blacktail Rattlesnake (Crotalus molossus nigrescens). Also known as palance and tlehua locally. I made a comment on your other pic of the youngster. If it was from the same area it is also probably C.m. nigrescens instead of C.m. basiliscus. The two forms intermix at mid to high elevations along the west coast of Mexico and are often difficult to distinguish. IMG_2487 | Flickr - Photo Sharing!