CHAPTEE V. 



SCALED KEPTILES, Order SQUAMATA continued ; THE SNAKES, 



Suborder OPHIDIA. 



Distinctive ORIGINALLY regarded as representing a distinct ordinal group of 

 Characters and the Reptilian class, the snakes are now generally considered to form 

 structure. me rely a suborder of the great assemblage of reptiles which includes 

 both lizards and chamseleons ; and from their close structural resemblance to the 

 limbless lizards there can be no doubt that the more modern view is the true one. 

 As a matter of fact, it is by no means easy to draw a satisfactory distinction be- 

 tween lizards and snakes ; and such characters as naturalists rely on for their 

 differentiation are mainly such as are due to adaptation to the special needs of the 

 latter group. Agreeing with lizards in their external covering of scales, snakes are 

 characterised by their exceedingly elongated and slender bodily conformation ; the 

 head, which is generally more or less flattened, being often not defined from the 

 body by a distinctly marked neck, while external limbs are wanting, and the body 

 passes posteriorly by imperceptible degrees into the tail. Occasionally, however, 

 external vestiges of the hind-limbs may persist, in the form of a pair of small spur- 

 like processes near the vent ; and internally there may be traces not only of the 

 pelvis, but likewise of- the thigh-bone, or femur. None of these characters will, 

 however, serve to distinguish snakes from the limbless lizards ; and it is there- 

 fore necessary to point out how the two groups may be separated. The most 

 characteristic peculiarity of snakes, as distinct from lizards, is the absence of a 

 solid union between the two branches of the lower jaw, which are connected 

 at the chin merely by an elastic ligament; this arrangement permitting of the 

 separation of the two halves of the jaw, and thus allowing the mouth to be dilated 

 so as to be capable of swallowing prey of much larger dimensions than the normal 

 width of its aperture. In addition to this arrangement, in the majority of snakes the 

 bones of the upper jaw and palate are likewise movable, thus further increasing 

 the capacity of the gape. From the great majority of lizards snakes are, of course, 

 widely distinguished by the absence of functional limbs ; while from the compara- 

 tively few limbless representatives of the same suborder, they differ in having 

 the tongue completely retractile within a basal sheath, as well as by the presence 

 of additional articulations to the vertebrae, which are described below. Moreover, 

 none of the limbless lizards have the large shields on the inferior surface charac- 

 terising the majority of snakes, while most of them possess eyelids and an 

 external aperture to the ear. 



No snake has movable eyelids ; that portion of the skin representing the lids 

 extending as a convex transparent disc across the eye, and covering it as a watch- 



