CHAPTER V 



SCALED REPTILES ORDER SQUAMATA continued; THE SNAKES 



SUBORDER OPHIDIA 



ORIGINALLY regarded as representing a distinct ordinal group of 

 Distinctive t ^ e Reptilian class, the snakes are now generally considered to form 



C aracter- mere ^ a SUDO rder of the great assemblage of reptiles which includes 

 istics and 



Structure ^ ot ^ h' zar ds and chamseleons; and from their close structural resem- 

 blance 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 between lizards and snakes; and such characteristics 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 characterized 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 characteristics will, however, serve to distinguish snakes from the 

 limbless lizards, and it is therefore 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 dimen- 

 sions than the normal width of the 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 comparatively few limbless representatives of the same sub- 

 order, 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 characterizing 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 



(2535) 



