ROOM X.] 
NATURAL HISTORY. 
43 
have long conical tails, and broad plates under the ab¬ 
domen. These species are, in general, innocent; a few 
have some of the upper lateral teeth rather larger than the 
rest, and grooved on the hinder edge, the groove communi¬ 
cating with a gland placed on the side of the face, but 
their bite is seldom so dangerous as that of the eminently 
poisonous snakes. The species of this division are ex¬ 
ceedingly numerous and difficult to determine, and they 
have lately been divided into many genera, which it 
would be tedious to characterize in this sketch. Such 
of them as live on the ground and take to the water 
for protection, or to catch their food, have generally a 
cylindrical form, and a tail scarcely as long as the body; 
while those which live the greater part of their life on 
trees, and are thence called Tree-Snakes (Dendropheis), are 
generally very long and slender, and the scales on the 
sides of the back usually narrow, and longer than those on 
the dorsal line : some of the Tree-Snakes have the end of 
the muzzle lengthened out into an acute appendage ( Pas - 
seritci). 
The Bull-headed Snakes ( Dipsades) resemble the Tree- 
Snakes in form, but the head is short and broad, the body 
compressed, and the latter has a series of larger scales down 
the back. In this group the fangs are most commonly 
found intermingled with the teeth, in which character they 
agree with the Cerberi {Homolopsies'), which are easily dis¬ 
tinguished from all the other snakes by the head being 
scaly, with a few small plates over the face and between 
the eyes. 
The Boas have usually a short body, with narrow plates 
on the abdomen, and a short conical tail, furnished with 
two short crooked spurs at its base. These spurs have 
lately been shewn to be analogous to the hinder legs of 
other reptiles. The Boas are not venomous; they kill 
their prey by crushing it between the folds of the body, 
generally, at the same time, twisting the end of the tail round 
a tree, in order to increase their power. 
The American species ( Boce ) have only a single row of 
plates beneath the tail; they vary greatly in the structure 
of the scales on the head and lips. 
Some of the Indian species, as the Netted Boa (Boa 
regia), have a single series of plates, whilst most of the 
