I. No transverse (ectopterygoid) bone; pterygoid not extending to quadrate or mandible; no supratemporal; nasals in contact with prefrontals; coronoid present; vestiges of pelvis. Maxillary loosely attached to lower surface of cranium, toothed; lower jaw edentulous; a single pelvic bone 1. TYPHLOPIDÆ. Maxillary bordering mouth, forming a suture with premaxillary, prefrontal, and frontal, toothless; pubis and ischium present, latter forming a symphysis 2. GLAUCONIIDÆ. II. Transverse bone present; both jaws toothed. _A._ Coronoid present; nasals in contact with prefrontals. 1. Vestiges of pelvis; supratemporal present. Supratemporal large, suspending quadrate 3. BOIDÆ. (Subfamilies: _Pythoninæ_, _Boinæ_.) Supratemporal small, intercalated in the cranial wall 4. ILYSIIDÆ. 2. No vestiges of pelvis; supratemporal absent 5. UROPELTIDÆ. _B._ Coronoid absent; supratemporal present. 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. (Subfamilies: _Viperinæ_, _Crotalinæ_.) The technical terms employed in the above synopsis will be found explained and illustrated by figures in the chapter on the Skeleton. No serial arrangement can express the affinities of the various groups as conceived by the classificator; a diagram therefore follows to show the author's views as to their interrelationships, and possibly their phylogeny. Leaving aside the Typhlopidæ and Glauconiidæ, which should be regarded as burrowing types independently derived from some Ophidian form less specialized than any with which we are at present acquainted, and probably without direct relationship to the Lizards, the family Boidæ, and more especially the Pythons, claim the position of ancestral group, from which all other snakes may have been derived.