Boulenger's phylogenetic system stands as follows: Viperidae Uropeltidae C. Opisthoglypha C. Proteroglypha Amblycephalidae mandible to the aglyphous or innocuous Colubridae, whence further differentiation in three new lines has taken place, - (i) the harmless Amblycephalidae as a side-issue, (2) the very poisonous proteroglyphous Elapidae, (3) the moderately or incipiently poisonous Opisthoglypha, out of some of which seem to have arisen the venomous Viperidae.
Viperidae.-The maxillaries are very short, movably pivoting upon the prefrontals and also attached to the ectopterygoids, so that they can be erected together with the large poison fangs, which, besides reserve teeth, are the only maxillary teeth.
All the Viperidae are very poisonous and all, except the African Atractaspis, are viviparous.
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