212 VERTEBRATE ANIMALS OF THE UNITED STATES 



being too small to be dangerous) ; top of head covered with symmetrical 

 plates; a single row of ventrals and usually a double row of subcaudals 

 present ; no rudimentary hind limbs ; eyes well developed : i ,300 species, 

 in all the tropical and temperate regions of the world; over 100 species 

 in the United States. 



Key to the United States Genera of Colubridae 



ai Scales keeled. 



bj Snout turned up at the end 5. Heterodon. 



ho Snout flattened and truncated at the end 9. Phyllorhynchus. 



hz Snout normal. 



Ci Anal plate divided. 



di Color bright green 7. Opiicodrys. 



d2 Color not green. 



Ci Scales in 15 to 17 rows; size small. 



fi No loreal present 28. Storeria. 



£2 Loreal present; no preocular. 



gi One internasal present 29. Potamophis. 



g2 Two internasals present 30. Amphiardis. 



e2 Scales in 19 or more rows. 



fi Ventrals 200 or more; belly flattened, with 



a ridge along each side 11. Elaphe. 



i-i Ventrals less than 165 25. Natrix. 



cj Anal plate single. 



di One nasal present on each side 27. Tropidoclonion. 



6.2 Two nasals present on each side. 



Ci Scales in ig to 21 rows; garter snakes 32. Thamnophis. 



e2 Scales in 25 or more rows; size large 13. Pituophis. 



3.2 Scales smooth. 



bi Anal plate divided. 

 Ci Scales in 13 rows. 



di In the central and eastern States i. Carphophis. 



do On the Mexican border 24. Chilomcniscus. 



C2 Scales in 15 to 17 rows. 



di Color black, with a yellow collar; size small. ... 4. Diadophis. 

 d2 Without such markings. 



Ci Color pale green; size small 6. Liopcltis. 



cs Color not green. 



fi Size large; ventrals 180 to 200. 



gi Snout truncated; 3 longitudinal stripes. 8. Salvadora. 

 g2 Snout not truncated; mostly without 



stripes or spots 10. Coluber. 



fo Size medium or small; ventrals 125 to 160. 

 gi Ventrals about 125. 



hi Upper labials 6 31. Virginia. 



h2 Upper labials 7. 



ii In the southeastern States 15. Lcimadophis. 



