REPTILIA. 263 
a vertical plane, the posterior of which being used as a fulcrum, or point 
dappui, the straightening of the anterior must result in the advance of the 
head, which in turn is fixed while the rest of the body is again flexed. 
The same condition may also prevail where the undulations are horizontal, 
and the snake constantly in contact with the ground. The most rapid 
movements, in all probability, are those occurring when the whole body is 
gathered up into one vertical loop like a bent spring, the head and tail 
more or less approximated : the sudden straightening of this loop or spring, 
with the tail as the point dappui, might enable the animal to spring 
forward, at one operation, to a distance greater than the length of its body. 
The great flexibility of their bodies enables serpents to obtain access to the 
most varied situations, by climbing or otherwise. Many species can climb 
trees in search of their prey, while others live habitually in such situations. 
Others are as constant inhabitants of the water. 
The phenomena of reproduction are different in different species. It 
may, perhaps, be considered as a general rule, that most of the venomous 
serpents are ovo-viviparous. This, however, with some appears in a 
measure to depend upon the latitude and mean temperature. Some harm- 
less species, again, are ovo-viviparous, as most of the North American 
Tropidonoti. Providence has taken the usual precautions against the 
increase of dangerous animals by assigning a small number of young to the 
venomous species. Thus the rattlesnake (Crotalus durissus) rarely pro- 
duces over nine or ten at a birth, while in one instance, 81 living garter- 
snakes (Tropidonotus sirtalis), of over nine inches in Jength, have been 
taken from a single individual. 
But few species of Ophidia have been found in a fossil state. None 
- from North America have been described; some of their remains have 
been procured in the bone caves of Pennsylvania. All that are known 
~ belong to the tertiary epoch. Remains of a species, 20 feet in length, have 
been found in the London clay at Sheppey. Doubtful indications of fossil 
Crotali exist in the vicinity of Brussels. 
A scientific exposition of the Ophidia, according to their natural affini- 
ties, is a matter of considerable difficulty, as the recent discovery of 
numerous new species, and the obscurity which hangs over many of the 
old, have completely unsettled the older views on this subject. In no 
other department of Zoology have the views of systematists been more at 
variance with each other than in that of Ophiology ; the important labors 
of Oppel, Fitzinger, Bonaparte, Schlegel, Gray, and others, only serving to 
render this truth more conspicuous. We shall, with J. E. Gray, divide the 
order into five families: Crotalide, Viperide, Hydride, Boide, and Colu- 
bride ; the two first arranged under a sub-order Viperina, the remaining 
three under Colubrina. 
Sub-order 1. Viperina. 
This sub-order includes most of the species which, on account of their 
venomous properties, have been the terror of mankind. They are dis- 
467 
