214 THE SNAKES OF SOUTH ‘AFRICA. 
Apart from the outward form, the Viperine snakes differ from 
the Colubrines in the formation of the upper jaw bones. The 
maxillary bones in which the fangs are set are mobile, allowing 
erection and depression of the poison fangs, which are large, 
re-curved and hollow, like the needle of a hypodermic syringe. 
Behind the front fangs several duplicate sets of fangs are stowed 
away, graded in size down to mere specks. These are the Viper’s 
reserve set of weapons. If the front active pair of fangs be 
shed or lost accidentally, the first pair of the reserve set take their 
place, and become connected with the ducts communicating 
with the poison glands. The supply of these reserve fangs is 
indefinite. If those which are visible are removed, others develop. 
NUMBERS AND DISTRIBUTION. 
There are about forty-two species of Vipers, and sixty-four 
species of what are known as Pit-Vipers. The Vipers inhabit 
Africa, Europe and Asia. The Pit-Vipers are unknown in Africa, 
but occur all over Southern Asia, Central and South America. 
The Rattle-snakes of America are Pit-Vipers. They are termed 
such because of a pair of deep pits in the fore part of the head, 
between the eyes and nostrils. The Pit-Vipers are classified 
under the Sub-Family Crotaline. In South Africa we have 
ten species or kinds of Vipers. 
SYNOPSIS OF THE GENERA. 
GENuS Causus.—Head distinct from neck, covered with symmetrical 
shields; nostril between two nasals and the internasal; loreal 
present ; eye moderate, with round pupil, separated from the 
labials by suboculars. Scales smooth or keeled, with apical 
pits, oblique on the sides, in 15 to 22 rows; ventrals rounded. 
Tail; short, sub-caudals in two rows or single. 
Genus Bitis.—Head very distinct from neck, covered with small im- 
bricate scales; eye moderate or rather small, with vertical 
pupil, separated from the labials by small scales; nostrils 
directed upwards, or upwards and outwards, pierced in a single 
or divided nasal, with a deep pit or pocket above, closed by a 
valvular, crescentric supranasal. Postfrontal bone very large, 
in contact with the ectoptergoid, which has an outer, hook- 
shaped process. Scales keeled, with apical pits, in 22 to 41 rows ; 
lateral scales in some species slightly oblique; ventrals rounded. 
Tail very short ; sub-caudals in two rows. 
Genus ATRACTASPIS.—Poison fangs enormously developed ; a few teeth 
on the palatines, none of the pterygoids. Head small, not dis- 
tinct from neck, covered with large symmetrical shields. Nostril 
between two nasals; no loreal. Eye minute, with round pupil. 
Postfrontal bone absent. Body cylindrical. Scales smooth, 
without pits, in 17 to 37 rows; ventrals rounded. Tail short. 
