Black-tailed Rattlesnake

Crotalus molossus, (B. & G.)

Colouration: The ground-colour above is rich sulphur-yellow, yellowish-brown, or olive. Down the back is a series of brown rhombs, with a narrow, yellow border, and open at the sides, whence they extend to the abdomen as narrow bands. Each of the rhombs usually contains two yellowish blotches. Toward the tail the markings become obscure. The tail is uniform black. (Occasional specimens are said to have the tail alternately barred with the ground-colour and black.)
Dimensions: The largest specimen examined by the writer was forty-nine inches long. It was fully adult, with a rattle consisting of nine, perfectly equal segments, and was captured in the mountains near Tucson, Arizona.
Distribution: In the United States the range of this species extends but a slight distance north of the Mexican boundary--not beyond the central portion of Arizona and as far north as this it is rare.
Habits: Like many of the rattlesnakes this species frequents rocky places, lurking in the vicinity of the mountain ledges where the numerous fissures afford it shelter.

By the uniform jet-black of the tail, this fine Rattlesnake is readily distinguished from the other species occurring in the Southwest. It attains a fair length--from three and a half to five feet, and is proportionately stout of body. The head is large, and quite blunt at the snout. On the upper portion of the snout are three pairs of enlarged scales or shields.

With the exception of a brown blotch or band on each side of the head, behind the eye, there are no head markings.

Following are the measurements of a young specimen captured in Mexico, a short distance over the border and south of Arizona; although these measurements are below that of the adult, they show the general proportions:

Total Length . . . . . 20 inches.

Length of Tail, not including rattle . . . . . 1 inches.

Greatest Diameter . . . . . 3/4 inches.

Width of Head . . . . . 13/16 inches.

Length of Head . . . . . 1 1/8 inches.

The specimen had a perfect rattle of five segments and, judging from the size of the basal ring, was about two-thirds grown.

North of the Mexican boundary it occurs from the Pecos River in Texas to slightly more than the central portion of Arizona. The most northerly record of capture in New Mexico appears to be Fort Bayard. Two specimens were captured by Dr. E. Coues on San Francisco Mountain, in Arizona. (Stejneger, Report of U. S. National Museum, 1893, p. 426.)

In New Mexico and Arizona, immediately adjacent to the Mexican border the species seems to be common. The writer has received several skins from Tucson, Arizona. Following is a paragraph from a letter from Mr. Herbert Brown, of Yuma, Arizona: "The Black-tailed rattlesnake is fairly common in the Santa Catalina and Rincon Mountains, near Tucson."

The range of this snake into Mexico is not definitely known.