Reptiles

The long limbs which are necessary for a bipedal cursorial life, involve elongated feet. These are secured by lifting the heel en tirely off the ground, so that the animal walks on the ends of the metatarsals, the toes stretching out along the ground as they do in birds. As the foot is placed directly under the body it tends to become symmetrical about the middle third toe, and rapidly becomes either functionally or actually tridactyl. The hand, which serves as a grappling hook for catching the prey, is reduced to three fingers, all provided with claws, that on the thumb becoming very large indeed in the latest forms.

The other group of dinosaurs, the Ornithischia, pursue a some what similar course of modifications; they also become bipedal, some of them secondarily returning to a quadripedal life. But in them the extension of the pubis and ischium into long downwardly directed rods, which is necessary to afford suitable muscle attach ments, takes place in such a way that the pubis acquires two branches, one directed downward and forward, the other directed backward so that it lies parallel to the ischium. The early stages of this arrangement are not known, but it persists throughout the whole group.

Flight.

One group of lizards, the genus Draco, has the habit of living in trees and of passing from tree to tree by making great leaps, whose length is extended by the presence of a parachute, made by flaps of skin which project from the sides of the body and are supported by the much elongated ribs. Such gliding is scarcely flight in any true sense, it cannot be maintained by any action of the animal whilst in the air, and its extent is limited by the speed acquired at the original jump and by the height of the point of departure. A similar gliding habit, carried out without any elaborate mechanism by a mere concavity of the ventral surface, is exhibited by certain arboreal snakes from Borneo. The only reptiles which have acquired true flight were the extinct Pterodactyls.

There are two series of the animals, in one of which the tail is extremely short and probably functionless, whilst in the other the tail is a very long stiff rod bearing a horizontal fin at its hinder end. The presence of this fin renders the maintenance of the body on an even keel much easier than it can have been in the tailless forms. The wing of every pterodactyl consists of a fold

of skin which is supported by the greatly extended fourth finger and by the hind leg; it may or may not have connected the hind legs together, either directly or by passing on to the base of the tail. The structure of the fore limbs, which enabled these animals to perform automatically the many carefully-adjusted movements which are necessary for flight, is so strictly determined by me chanical considerations that it is practically uniform in all known species of the group.

In all of them, the scapula is an elongated narrow rod of bone which may articulate with the neural spine of the dorsal verte brae. Its lower end forms the upper half of the glenoid cavity and is fused with the coracoid. This bone is elongated and straight, its distal end rests in a groove in the front of the large sternum, so that it is enabled to take directly the stresses resulting from the powerful wing muscles. The humerus is short, and the radius and ulna lie parallel to one another. The carpus consists of three bones, with the distal of which the wing metacarpal is articulated, so that it can revolve on its axis. The main joint at which the wing was folded lay between the metacarpal and first phalanx. The movements at the elbow and wrist are inseparably connected, and serve to alter the camber and angle of attack of the wing, thus enabling the animal to fly at varying speeds. It is interesting to note that a successful aeroplane, with an unusual range of flying speed, has been designed on lines suggested by the tailless pterodactyls.

Swimming.

Two extreme modes of swimming are open to a tetrapod. It may convert its limbs into paddles by whose actions it rows itself through the water, or it may use its tail as a pro peller, either flattening it and causing waves to pass along its length or producing a fin at the extreme tip, which can be used like a screw propeller. Both types are found in reptiles.

The Chelonia include amongst the fresh water tortoises a num ber of animals which swim well with limbs which, except for the webbing of the toes, are much like those of a land animal. But in the marine turtles and in the fresh water Carettochelys from New Guinea, the limbs are transformed into paddles, mere bags of skin surrounding the whole of the elongated digits.