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The snakes belonging to this genus of Crotalinæ are found in Central and Eastern Asia, but three important species occur in the New World, in the United States and Central America. The head is covered with nine large symmetrical shields, but the internasals and præfrontals are sometimes broken up into scales. The body is cylindrical; the tail rather short; the subcaudal scales are arranged in one or two rows.

A. acutus.—The snout of this snake is prolonged into an appendage directed forwards. The head-shields are finely granulate. Body scales arranged in 21 rows; 162–166 ventrals; 58–60 subcaudals.

Coloration brown on the back, with blackish-brown X-shaped spots; head dark brown, yellow on the cheeks, with a black band running from the eye to the angle of the jaw; belly yellowish, spotted with brown, with a series of large black transverse blotches.

Total length, 1,500 millimetres; tail 200.

Habitat: Upper Yang-tse, China.

A. halys.—Snout prolonged into an upturned appendage, blunt at the tip; 7–8 supralabial scales, the third of which forms part of the margin of the eye; body scales in 23 rows; 149–174 ventrals; 31–44 subcaudals.

Coloration yellowish, grey, red, or pale brown above, with darker spots or cross-bars. A black spot on the snout; two black spots on the vertex; a dark, light-edged band on the temple; belly whitish, more or less speckled with grey or brown.

Total length, 490 millimetres; tail 65.

Habitat: From the borders of the Caspian Sea and the Ural River to the Upper Yenisei; Turkestan.

A. intermedius.—Resembles the foregoing very closely, but the snout is not turned up at the end.

Total length, 750 millimetres; tail 85.

Habitat: Central Asia, Eastern Siberia, Mongolia, and Japan.

A. blomhoffii.—Similar to A. halys, but the snout is not turned up at the end, and the body scales are in 21 rows; 137–166 ventral scales; 29–55 subcaudals.

Coloration very variable; grey, brown, or red above, with large black-edged spots arranged in pairs; black, light-edged band on the temple; upper lip uniformly yellow or red; belly yellow or reddish, more or less spotted with black, or almost entirely black.

Total length, 720 millimetres; tail 100.

Habitat: Eastern Siberia, Mongolia, China, Japan, Siam.

A. himalayanus.—Snout scarcely turned up, with a hard tip; 5–7 supralabial shields. Body scales in 21 (rarely 23) rows; 144–166 ventrals; subcaudals in 35–51 pairs.