1. Maxillary horizontal; pterygoids reaching quadrate or mandible.
Nasals in contact with prefrontals
6. Xenopeltidæ.
Nasals not in contact with prefrontals
7. Colubridæ.
Three series: A. Aglypha (subfamilies: Acrochordinæ, Colubrinæ, Dasypeltinæ); B. Opisthoglypha (Homalopsinæ, Dipsadomorphinæ, Elachistodontinæ); C. Proteroglypha (Hydrophiinæ, Elapinæ).
2. Maxillary horizontal, converging posteriorly towards palatine; pterygoid not reaching quadrate or mandible
8. Amblycephalidæ.
3. Maxillary vertically erectile perpendicularly to transverse bone; pterygoid reaching quadrate or mandible
9. Viperidæ.