This order includes a group of extraordinary flying Reptiles, all belonging to the Mesozoic epoch, and exhibiting in many respects a very extraordinary combination of characters. The most familiar members of the order are the so-called "Pterodactyles," and the following are the characters of the order:

No exoskeleton is known to have existed. The dorsal vertebrae are procoelous, and the anterior trunk-ribs are double-headed. There is a broad sternum with a median ridge or keel, and ossified sternal ribs. The jaws were generally armed with teeth, and these were implanted in distinct sockets. In some forms (Ram-phorhynchus) there appear to have been no teeth in the anterior portion of the jaws, and these parts seem to have been sheathed in horn, so as to constitute a kind of beak. In the genus Pteranodon, from the Cretaceous rocks of North America, comprising gigantic examples of the order, the jaws are completely destitute of teeth, and appear to have been encased in a horny beak.

A ring of bony plates occurs in the sclerotic coat of the eye. The pectoral arch consists of a scapula and distinct coracoid bone, articulating with the sternum as in Birds, but no clavicles have hitherto been discovered. The fore-limb (fig. 317) consists of a humerus, ulna and radius, carpus, and hand of four fingers, of which the inner three are short and unguiculate, whilst the outermost is clawless and is enormously elongated. Between this immensely-lengthened finger, the side of the body, and the comparatively small hind-limb, there must have been supported an expanded fiying-membrane, or "patagium," which the animal must have been able to employ as a wing, much as the bats of the present day. Lastly, most of the bones were "pneumatic" - that is to say, were hollow and filled with air.

Fig. 317.   Pterodactylus brevirostris. Skeleton and restoration.

Fig. 317. - Pterodactylus brevirostris. Skeleton and restoration.

By the presence of teeth in distinct sockets, and, as will be seen hereafter, especially in the structure of the limbs, the Pterodactyles differed from all known Birds, and there can be little question as to their being genuine Reptiles. The only Reptiles, however, now existing, which possess any power of sustaining themselves in the air, are the little Dragons (Draco), but these can only take extended leaps from tree to tree, and cannot be said to have any power of flight properly so called. That the Pterodactyles, on the other hand, possessed the power of genuine flight, is shown by the presence of a median keel upon the sternum, proving the existence of unusually developed pectoral muscles; by the articulation of the coracoid bones with the top of the sternum, providing a fixed point or fulcrum for the action of the pectoral muscles; and, lastly, by the existence of air-cavities in" the bones, giving the animal the necessary degree of lightness. The apparatus, however, of flight was not a "wing," as in Birds, but a flying membrane, very similar in its mode of action to the patagium of the Mammalian order of the Bats. The patagium of the Bats, however, differs from that of the Pterodactyles in being supported by the greatly-elongated fingers, whereas in the latter it is only the outermost finger which is thus lengthened out. The difficulty as to the position of the Pterosauria is evaded by Mr Seeley by placing them in a distinct class, which he terms Ornithosauria, and which he regards as most nearly related to, but coequal with, the class Aves.

The Pterosauria are exclusively Mesozoic, being found from the Lower Lias to the Chalk inclusive, the Lithographic Slate of Solenhofen (Upper Oolite) being particularly rich in their remains. Most of them appear to have attained no very great size, but the remains of a species from the Cretaceous rocks have been considered to indicate an animal with more than twenty feet expanse of wing, counting from tip to tip.

In the genus Pterodaclylus proper, the jaws are provided with teeth to their extremities, all the teeth being long and slender.

In Dimorphodon, the anterior teeth are large and pointed, the posterior teeth small and lancet-shaped.

In Ramphorhynchus, the anterior portion of both jaws is edentulous, and may have formed a horny beak, but teeth are present in the hinder portion of the jaws.

In Pteranodon, lastly, the jaws are completely edentulous, and were probably ensheathed in horn. This genus, along with some small forms, includes the largest known members of the order.