(2) _S. catenatus_ (Prairie Rattle-Snake, or Massasanga, fig. 71).--Two or three series of scales between the eye and the labials; 11-14 supralabials; scales in 23 or 25 rows; 136-153 ventrals; 20-31 subcaudals. Colour the same as in _S. miliarius_: a dark spot on the parietal shields. Total length, 680 millimetres; tail 80. _Habitat_: Great Lakes district; United States east of the Rocky Mountains and west of the Mississippi; Northern Mexico. (3) _S. ravus._--11 or 12 supralabials; scales in 21 or 23 rows; 147 ventrals; 26 subcaudals. Colour yellowish-brown, with a dorsal series of dark brown spots, longer than broad, and a series of transverse dark bars on each side; belly yellowish, spotted with blackish-brown. [Illuatration: FIG. 71.--_Sistrurus catenatus_ (Prairie Rattle-Snake, or Massasanga). (After Holbrook and Stejneger.)] Total length, 200 millimetres; tail 22. _Habitat_: Vera Cruz, Mexico. (d) =Crotalus= (Rattle-Snakes). These snakes differ from all others in that the end of the tail bears a series of large conical scales, forming rattles, each fitting into the next and movable in such a manner that when the reptile causes them to move they produce a strident sound (fig. 72). [Illustration: FIG. 72.--A, Horny appendage (rattle) of _Crotalus horridus_ (three-quarters natural size. After Garman). B, longitudinal section of the same. C, separated segments of the appendage: _a_, terminal point; _h_, basal segment (after Czermak).] The length of these animals often exceeds 2,000 millimetres. The head is flat, very large and expanded posteriorly, and terminated in front by a short, truncate snout; it is covered above with scales or small shields. Rattle-Snakes are armed with enormous fangs enclosing a complete canal, which extends throughout almost their entire length. The poison-glands are of the size of a large almond. The number of segments in the rattle is variable, but rarely exceeds 18 or 20. At the time of the shedding of the skin these segments fall off and are at once replaced. Contrary to the belief which was long entertained, their number bears no relation to the age of the snake. [Illustration: FIG. 73.--_Crotalus terrificus_ (Dog-faced Rattle-Snake, Cascavella in Brazil). (After Stejneger.)] Rattle-Snakes are met with especially in stony and arid localities, or among brushwood near water. They hardly ever bite except when surprised or attacked. (1) _C. terrificus_ (Dog-faced Rattle-Snake, _Cascavella_ in Brazil; fig. 73).--Snout very short; three or four series of scales between the eye and the supralabial shields; body-scales in 23-31 rows, dorsals very strongly keeled; 160-199 ventrals; 18-30 subcaudals. Colour brown, with a series of darker, light-edged rhombs, often lighter in the centre; a dark streak from the eye to the angle of the mouth; belly yellowish-white, uniform or spotted with brown; tail generally brown or blackish. Total length, 1,320 millimetres; tail 130.