EXTINCT OEDERS OF REPTILES. 227 



flight is conclusively shown by the presence of a median 

 keel upon the sternum, proving the existence of unusually- 

 developed pectoral muscles ; by the articulation of the cora- 

 coid 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, 

 this being a feature otherwise peculiar to the true Birds. 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. Moreover, to mention one 

 other point of difference only, the Pterodactyles possess 

 pneumatic foramina in some of the bones, indicating a 

 structure of the breathing-oroans similar to that now found 

 in the Birds, and wholly unknown amongst the Mammals. 

 The only question, then, at the present day is as to whether 

 the Pterosaurs are most nearly related to the Eeptiles or to 

 the Birds ; and it is amongst the former that they are most 

 generally placed, No known Eeptile has any power of sus- 

 taining itself in the air in any manner which can justifiably 

 be compared with the flight of Birds ; since the little Fly- 

 ing Dragons {Draco) simply take leaps from tree to tree by 

 means of laterally-extended folds of skin. No known Eeptile, 

 further, has pneumatic bones ; and there are other points of 

 difference which separate the Pterosaurs from all the typi- 

 cal Eeptiles. Still, the general structure of the skeleton is 

 distinctly Eeptilian ; and the absence of a non-conducting 

 covering of feathers to the skin would prove that the ani- 

 mal must have been cold-blooded. The structure of the 

 hand, further, though abnormal, is exceedingly unlike that 

 which obtains in Birds. Lastly, it is only in certain very 

 aberrant Cretaceous Birds that we meet with teeth in the 

 jaws. These considerations would seem to justify the ref- 

 erence of the PtcTosauria to the Reptilia, of which they 

 form an altogether peculiar order. Prof. Seeley, however, 

 regards the Pterosaurs as formino- a distinct class which he 



