REPTILES 231 



Key to the United States Genera of Crotalidae 



ai No rattle present i. Agkistrodon. 



a2 Rattle present. 



bi Top of head covered with plates (Fig. 126) 2. Sistrurus. 



hi Top of head covered with scales (Fig. 127) 3. Crotalus. 



I. Agkistrodon Beauvois. Head covered with 9 symmetrical plates; 

 no rattle present; scales in 23 or 25 rows; upper labials 8; ventrals 130 to 

 150; anterior subcaudals undivided, posterior ones divided: 10 species, 

 7 in Asia, 2 in the United States. 



Key to the Species of Agkistrodon 



ai Scales in 23 rows; loreal plate present A. mokasen. 



a2 Scales in 25 rows; loreal plate absent A. piscivorus. 



A. mokasen Beauvois {A. contortrix L.). Copperhead; moccasin 

 (Fig. 124). Length 830 mm.; tail 120 mm.; color light brown, with 

 about 15 transverse bands of darker brown which are narrow on top and 

 very broad at the sides; head copper color; belly whitish, with a series 



Fig. 124. — Agkistrodon mokanse (from Fig. 125 — Agkistrodon piscivorus (Jrom 



Cope) . Cope) . 



of large dark spots on each side: Massachusetts to northern Florida; 

 westward to Illinois and Texas; sometimes abundant in swampy 

 or rocky places, feeding on frogs, rodents, birds and snakes; bite very 

 dangerous. 



A. piscivorus (Lacepede). Water moccasin; cottonmouth (Fig. 

 125). Length 1,000 mm.; tail 190 mm.; color olive brown, crossed by 

 20 to 30 obscure darker bands; head purplish black; belly yellow, with 

 black blotches: Atlantic and Gulf drainage from Virginia to eastern 

 Texas; semiaquatic, often common along streams, feeding on frogs, 

 fish, rodents and birds, for which the snakes will often He in wait, 

 perched in bushes overhanging the water; bite very dangerous. 



2. Sistrurus Garman. Rather small snakes, with head covered 

 with 9 symmetrical plates, and with a small rattle composed of not 

 more than 8 rings; scales in 21 to 25 rows; upper labials 10 to 12; ven- 

 trals 125 to 145; subcaudals, except a few of the terminal ones, undi- 

 vided: 3 species, i in Mexico. 



