LOCOMOTION OF 'LIMBED REPTILES. 265 



straight-forward rush : this, for a short extent, is dangerously 

 rapid ; but the Crocodiles are most formidable and agile in their 

 habitual element the water. 



The little Draco volans has a body which rarely exceeds 

 110 grains in weight; the delicate tegumentary parachute, sustained 

 by the long slender ribs, fig. 163, like the nervures of the insect's 

 wing, measures about five square inches — its area being as great 

 in proportion to the weight of the animal as that of the wings in 

 many birds. But the muscular apparatus, a, a, subserves only 

 the expansion and folding up of the membrane, which would 

 seem, therefore, to act, if the animal ever leaps into air or darts 

 through that element, merely as a sustaining parachute to break 

 the fall. 



The Reptilia in which the fore limb was developed and modified, 

 in order to work membranous expansions with sufficient force to 

 raise and move the body in air, ceased to exist, apparently, during 

 the deposition of the cretaceous beds, prior to the tertiary epoch 

 in Geology. We learn from the fossilized remains of Ptero- 

 dactyles, that the weight of the body, compared to the area of 

 their outspread wings, must have been very small, fig. Ill, a; 

 that the bones were light, of a thin but compact osseous texture, 

 permeated by air. The digit, so enormously developed for 

 sustaining the main part of the wing, fig. Ill, 5, was restricted, 

 like the antibrachium in birds, to movements of abduction and 

 adduction, lying along the ulnar side of the fore-arm, and reaching 

 beyond the sacrum, when the wing was folded. The proportion 

 of the area of the outstretched wings to the body was greater in 

 Pterodactyles than in most birds, and equalled that in the bats ; 

 like which, the Pterodactyles would alter the alar area by alternate 

 abduction and adduction of the sustaining digit, combined with 

 flexion and extension of the arm and fore-arm. 



The large head and strong neck of the Pterodactyle seem to 

 have called for that extension and forward direction of the anti- 

 brachium, which would cause the centres of gravity and magnitude 

 to be more in advance than in either bird or bat. Their pelvic 

 limbs were little more developed than in Bats — must have been 

 unequal to sustain the body — may have concurred with the short 

 unguiculate digits of the fore limb, fig. Ill, l, 2, 3, 4, in a crawling 

 progress along the ground — and, being terminated by toes of 

 equal length, probably served, as in bats, to suspend the body, 

 head downward, during sleep. 



