210 



VERTEBRATE ANIMALS OF THE UNITED STATES 



About 1, 800 species of snakes are known, of which 126 species 

 occur in the United States, grouped in 5 famiUes. 



Fig. 115. — Head of snake {Coluber constrictor) showing the plates: i, frontal; 2, parietals 

 3, supraoculars; 4, prefrontals; 5, internasals; 6, preoculars; 7, loreal; 8, postocular; 9, ros- 

 tral; 10, upper labials; 11, temporals; 12, nasals; 13, lower labials (from Hay). 



Key to the Families of Serpentes 



ai No ventrals present, the belly being covered with scales; 



eyes covered with scales i. Leptotyphlopidce. 



a,2 Ventrals present; eyes not covered with scales. 



bi No elongate poison fangs in front part of mouth. 



Ci Subcaudals undivided, forming a single row 2. BoidcB. 



C2 Subcaudals divided, forming 2 rows (except in Rhino- 



cheilus) 3. Colubridce. 



h-i Elongate poison fangs present in front part of mouth. 



