PTEROSAURIA.
Char.—Pectoral members, by the elongation of the anti brachium and fifth digit, adapted for flight. Vertebre proccelian ; those of the neck very large, not exceeding eight in number ; those of the pelvis few and small. Most of the bones pneumatic. Head large ; jaws long, and armed with teeth.
The species of this order of reptiles are extinct, and peculiar to the mezozoic period. Although some members of the pre ceding order resembled birds in the shape or the edentulous state of the mouth, those of the present order make a closer approach to the feathered class in the texture and pneumatic character of most of the bones, and in the development of the pectoral limbs into organs of flight (fig. 74). This is due to an elongation of the antibrachial bones, and more especially to the still greater length of the metacarpal and phalangial bones of the fifth or outermost digit (fig. 74, s), the last phalanx of which terminates in a point. The other fingers were of more ordinary length and size, and terminated by claws. The number of phalanges is progressive from the first (fig. 74, i) to the fourth (4), which is a reptilian character. The whole osseous system is modified in accordance with the possession of wings ; the bones are light, hollow, most of them permeated by air-cells, with thin compact outer walls. The scapula and coracoid are long and narrow, but strong. The vertebrae of the neck are few, but large and strong, for the support of a large head with long jaws, armed with sharp Fossil skeleton of Pterodactultes crassirostrie : A, Sketch of living Pterodactyle.
pointed teeth. The skull was lightened by large vacuities, of which one (o, fig. 74) is interposed between the nostril n and the orbit 1. The vertebrae of the back are small, and grow less to the taiL Those of the sacrum are small, from three to five in number : but the weak pelvis and hind limbs bespeak a creature unable to stand and walk like a bird. The body must have been dragged along the ground like that of a bat. The Pterosauria may have been good swimmers as well as flyers. The vertebral bodies unite by ball-and-socket joints, the cup being anterior, and in them we have the earliest manifestation of the " proccelian" type of vertebra. The atlas consists of a discoid centrum, and of two slender neurapo physes ; the centrum of the axis is ten times longer than that of the atlas, with which it ultimately coalesces ; it sends off from its under and back part a pair of processes, above which is the transversely extended convexity articulating with the third cervical vertebra. In each vertebra there is a large pneumatic foramen at the middle of the side. The neural arch is confluent with the centrum. The anterior ribs have a bifurcate head. The dentition is thecodont.