80 
OK THE PHYSIOGKOMY OF SERPEKTS. 
animal suspended from a single point. The swimming 
Boas have not a different formation of the tail ; and it is 
only in the Acrochordus that it becomes slightly com^ 
pressed. The end of the tail is most frequently fortified 
by a simple conical scale, more or less pointed or hooked ; 
this point is converted in the Crotalus mutus into a hard 
spine ; but the other Crotali have the tail furnished with 
a peculiar rattle^ often very large, although it is but a 
simple production of the epidermis. 
The Head does not always present any correspondence 
in form with the other parts of the body. It is, for ex- 
ample, very compact and thick in the Dipsas, which, how- 
ever, has a very elongated body, as occurs also in the Den- 
dr ophis, although the head of the latter is very long and 
slender. Hence it may be perceived that the form of the head 
is chiefly influenced by the kind of food which nourishes 
the species. Those which swallow animals large in pro- 
portion to their own size have necessarily a large head, the 
parts of which can dilate, forming a contrast to what is 
found in those which live on worms, insects, or animals of 
small size or of slender forms. In such, the head is scarcely 
distinguishable from the trunk ; it is generally short, 
rounded, and thick at the muzzle, as in the Tortrix, the 
Calamaria, the Elaps, &c. In the first kind of snake, on 
the contrary, the head is very broad at the base, very dis- 
tinguishable from the trunk, and consequently very suscep- 
tible of an extraordinary degree of dilatation, as is especially 
the case with the venomous serpents, properly so called, 
and with several species of Dipsas, Xenodon, Boa, Coluber, 
&c* The Muzzle determines the general form of the head ; 
it is sometimes short and thick, sometimes rounded or 
truncated, at other times slender and pointed ; in some it 
terminates in a hard turned-up scale ; in others it is drawn 
out into a fleshy and moveable appendage. Sometimes, as 
in the Homalopsis Herpeton, we observe these appendages 
on each side of the snout ; but those which some Vipers 
have over the superciliary region are merely scales with 
pointed prolongations, more or less developed. The point 
of the muzzle always overlaps the lower jaw, the edges of 
which are lodged within those of the upper jaw ; but the 
