204 Trans. Acad. Sci. of St. Louis. 



be regarded as terminal ganglions, and it does not seem 

 doubtful that we have here to do with a true sense organ. 

 "Wherein this ' sixth sense' consists we do not know, 

 nor do we know of anything in the habits of these snakes 

 which would indicate its nature, or to what use the animal 

 puts the organ. Future research may reveal it, though 

 perhaps man will never fully comprehend the nature of 

 a sense which he himself does not possess." (Stejneger.) 



Key to the Genera of Crotalidae of Missouri. 



Tail not provided with a rattle. Agkistrodon. 

 Tail provided with a rattle. 



Top of head with plates. Sistrwrus. 



Top of head with scales. Crotalus. 



Genus agkistrodon. 



Nine cephalic plates. A pit between the eye and nostril. A pair 

 of erectable poison fangs. Scales keeled, in 21-27 rows. Anal entire. 

 No rattle. 



Key to the Species in Missouri. 



No loral plate; a pair of post-parietals; upper labials entering the 

 orbit. Usually 25 scale rows. piscivorus: 



A loral plate; no post-parietals; orbit separated from upper labials 

 by scales. Usually 23 scale rows. contortrix. 



80. Agkistrodon piscivorus LaCepede. Water Moccasin. 

 Cotton Mouth. 



Trigonoceplialus piscivorus, Toxicophis piscivorus, Scytale piscivorus, 

 Cenchris piscivorus, Toxicophis pugnax, Acontias leucostoma, Na- 

 trix piscivorus, Coluber aquaticus, Le Piscivore, Vipera aquatica. 



Description. — Rostral large, vertical, truncate above. Internasals tri- 

 angular, frontal large, longer than wide, hexagonal. Nasals two with 

 the nostril between them. A pair of small triangular plates behind 

 the parietals, which are pentagonal. Pre-oculars two or three, the 

 upper much the largest. Post-oculars two or three, with one or two 

 sub-oculars. Upper labials eight, the third the largest and entering 

 the eye. Ten lower labials. Anterior chin shields much larger than 

 the posterior. Head broad, flat on top, snout rounded. Canthus ros- 

 tralis sharp. Neck contracted. Body very stout. Tail short, com- 

 pressed, one-seventh to one-sixth of total length. Dorsal scales in 25 



