The poison-fangs are not grooved, as in the Proteroglyphous Colubridæ; they are pierced by a perfectly formed canal, the upper end of which inosculates with the efferent duct of the corresponding poison-gland, while its lower extremity opens to the exterior a little above and in front of the tip. The latter is always very sharp.
The palate and lower jaw are furnished with small hooked teeth, which are solid and non-venomous.
With the exception of the species of Atractaspis, these snakes are all ovoviviparous. The majority are terrestrial; a few lead a semi-aquatic existence, while others are arboreal.
Their distribution includes Europe, Asia, Africa (with the exception of Madagascar), and North and South America. They do not exist in Australia.
They are divided into two Sub-families:—
A. The Viperinæ, in which the head, which is very broad and covered with little plates and scales, has no pit between the nose and the eyes;
B. The Crotalinæ (κρὁταλον, a rattle), in which the head is incompletely covered with scales, and exhibits a deep pit on each side, between the eye and the nostril.
Among snakes, the characters that serve as a basis for the determination of genera and species are the general shape of the body, especially that of the head, the arrangement of the cephalic scales, the cranial skeleton, and the dentition.