ADAPTATIONS 1 1 1 



phisbaenas {Amphisbtznidce) ; fifthly, the Australian scale-foots 

 (Pygopodidce) ; and, sixthly, certain skinks, more especially 

 some members of the genera Chalcides and Lygosoma in the 

 family Scincidce, together with a few other degraded forms 

 which have been made the types of families. All these groups 

 except the first are members of the suborder Lacertilia, which 

 forms a division of the Squamata of the same rank as the 

 Ophidia. 



Snakes, as being the typical, and at the same time the most 

 numerous, "gliders," may be discussed first. The shape of 

 these reptiles is familiar to all. A special feature is the pre- 

 sence of large transverse scales, or shields, extending right across 

 the lower surface of the body, but on the tail frequently divided 

 into a double series. Each scale corresponds to a pair of ribs ; 

 and in gliding a snake advances the fore-part of its body, when 

 the scales on the lower surface are partially erected and take 

 hold of the ground or other surface in such a manner that the 

 rest of the body may be drawn forwards. As the ribs are the 

 active agents in this peculiar mode of progression, snakes may 

 be appropriately called rib-walkers. It should be added that the 

 movements of a snake on the ground are, with the exception of 

 the head and neck, confined to oscillations in a horizontal plane. 

 Certain snakes display unmistakable evidence of their deriva- 

 tion from reptiles of a more ordinary type by the retention, 

 either externally or internally, or both together, of vestiges of 

 the hind-limbs. In the pythons and boas (Bot'dcz) such exter- 

 nal vestiges take the form of a pair of small scale-like or spur- 

 like structures in the neighbourhood of the vent. In the 

 African Python sebce these vestiges form distinct claws ; but in 

 specimens of P. molurus twenty feet in length they are scarcely 

 visible. Somewhat similar but smaller vestiges also occur in 

 the coral-snake (Ilysia scytale) and the other members of the 

 family Ilysiidce. The internal vestiges, which occur both in the 

 Boida; and the burrowing Glanconiidcs, take the form of rem- 

 nants of the pelvis and occasionally also of the femur or thigh- 

 bone. In no instance does any trace of the fore-limbs persist, 

 thus showing that these were the first to disappear. Many 

 snakes, such the pythons and tree-snakes (Dendrophis) have 

 taken to an arboreal existence. Another group, the sea-snakes, 

 or Hydrophiincz, has, as already mentioned, taken to a pelagic 



