48 
POPULAR SCIENCE REVIEW. 
It is usual to distinguish four families of venomous snakes — 
the Crotalids or pit-vipers, the true vipers, the Elapidsand the 
Hydrophids or sea-serpents. 
The pit-vipers are, of all venomous snakes, the most modified 
in structure, those which depart furthest from the harmless * 
serpents. The long poison-fang is firmly fixed to the maxillary 
bone, which is extremely short and high, and moveable upon two 
other bones of the head. Behind the poison-tooth lie the 
rudiments of its successors, the so-called fangs of repl^icement. 
There are no other teeth in the maxillary bone. The sides of 
the poison-tooth are prolonged forwards and inwards to meet 
one another along the front of the fang, where they serve to 
enclose a tube, or canal, open near the point of the tooth for 
the escape of the poison, while, at its opposite end, the base of 
the fang communicates with the principal duct of the poison- 
gland. So large is the fang in proportion to the maxillary 
bone which supports it, that bofh might be said to constitute a 
single structure. The fangs share the movements of their 
bony supports and, when not in use lie hidden in the gum, 
along the roof of the mouth, with their points directed back- 
wards. As the mouth opens, the maxillce rotate sufficiently to 
erect the fangs, which are thus most favourably placed for at 
once inflicting a wound and discharging their venom.f Each 
maxillary bone has on its outer side a deep excavation, indi- 
cated externally by a conspicuous pit placed between the eye 
and nostril. The Crotalidae have a stout body, a moderate or 
rather short tail, and a broad triangular head, flattened above 
and very wide behind, passing suddenly into the neck. The 
tail is prehensile in some, not in others. The eyes are 
moderate, with vertical pupils. The head is variously covered 
with shields or scales, which supply characters used in defining 
the genera. These serpents are viviparous. 
Almost all the characters common to the Crotalidae are pre- 
sented, in a slightly reduced degree, by the true vipers, which 
want, however, the maxillary pit distinguishing the first-men- 
^ The words harmless and innocent, when used in the case of snakes, are 
synonymous with non^venomous. Most of these last are truly harmless to 
man ; not so, however, with the larger constricting serpents (Pythons and 
Boas), which, though not poisonous, are more to be dreaded than some 
species of Thanatophidia. 
t For a good account of the complex biting mechansim of serpents, which 
it is not our business here to describe at length, reference may best be made 
to Professor Huxley’s ‘^Anatomy of Vertebrated Animals,” pp. 238-241. 
'The reader, desirous further to study the anatomical characters of this 
order, should next consult the admirable Zootomie der Amphibien ” of Pro- 
fessor S tannins; 8vo, Berlin, 1856, 
