REPTILIA. 263 



a vertical plane, the posterior of which being used as a fulcrum, or point 

 (lappui, 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 d'appui, 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: srarter- 

 snakes (Tropidonotus sirtalis), of over nine inches in length, 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 : CrotalidcB, Vipeindce, Hydridoe, Boidce, and Colu- 

 bridce ; 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 



