(17) _L. undulatus._--Snout short, rounded; 11 supralabials; scales in 21 rows, the dorsals strongly or very strongly keeled; 149-171 ventrals; 41-49 pairs of subcaudals. _Tail prehensile._ Colour olive or brown, sometimes speckled with black, with a dorsal series of rhomboidal dark spots or an undulous or zigzag band; belly yellowish or brownish, powdered with blackish. Total length, 570 millimetres; tail 75. _Habitat_: Mexico. (18) _L. lateralis._--Snout rounded; 9 supralabials; scales in 21 or 23 rows, rather strongly keeled; 171 ventrals; 59 subcaudals, in a single row. _Tail prehensile._ Colour green, with a yellow line on each side of the body. Total length, 485 millimetres; tail 75. _Habitat_: Costa Rica. (19) _L. bicolor._--Very similar to the foregoing; scales in 21 rows; 164-167 ventrals; 62-67 subcaudals, in a single row. Colour uniform green, yellowish on the belly. Total length, 375 millimetres; tail 60. _Habitat_: Guatemala. (20) _L. schlegelii._--Snout rounded; 8 or 9 supralabials; scales in 19-25 rows, more or less strongly keeled; 138-162 ventrals; 47-62 subcaudals, in a single row. _Tail prehensile._ Coloration very variable, green or olive, spotted with black, or with pinkish, reddish, or purplish black-edged spots or cross-bars; belly yellow, spotted with green, or variegated; end of tail generally red. Total length, 600 millimetres; tail 115. _Habitat_: Central America, Colombia, Ecuador. (21) _L. nigroviridis._--Snout short, rounded; 9-11 supralabials; 19 rows of scales, rather feebly keeled; 134-146 ventrals; 49-54 subcaudals, in a single row. _Tail prehensile._ Colour green or olive, spotted with black; head with a black streak on each side; belly yellowish. Total length, 535 millimetres; tail 90. _Habitat_: Costa Rica. (22) _L. aurifer._--Snout short and broad; 9 or 10 supralabials; scales in 19 rows, rather strongly keeled; 154-158 ventrals; 53-61 subcaudals, single. _Tail prehensile._ Colour green, spotted with yellow; a black streak on the temple; belly greenish-yellow. Total length, 825 millimetres; tail 145. _Habitat_: Guatemala. (c) =Sistrurus.= Head very distinct from neck, covered above with nine large symmetrical shields; eyes rather small, with vertical pupils. Body cylindrical; scales keeled, with apical pits; tail short, ending in a segmented horny apparatus (rattle), producing a special sound; subcaudals all or the majority in a single row. (1) _S. miliarius_ (Ground Rattle-Snake).--9-11 supralabials; scales in 21 or 23 rows, strongly keeled; 127-139 ventrals; 27-36 subcaudals; rattle short, consisting at the most of 10 segments. Colour greyish, yellowish, or brown, the vertebral line often orange; two undulating dark stripes from between the eyes to the occiput, the enclosed space usually orange; belly whitish, spotted with dark brown or black. Total length, 520 millimetres; tail 70. _Habitat_: South-eastern North America, from North Carolina to Texas.