420 MANUAL OF ZOOLOGY. 



seem to have been sheathed in horn, so as to constitute a kind 

 of 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. 165) 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 flying-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. 



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 

 Reptile, however, now existing, which possesses any power of 

 sustaining itself in the air, is the little Draco volans, but this 

 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 exist- 

 ence of air-cavities in the bones, giving the animal the neces- 

 sary 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 Pterosauria are exclusively Mesozoic, being found from 

 the Lower Lias to the Middle 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 Pterodactylus proper, the jaws are provided 



