EXTIXCT ORDERS OF REPTILES. 



223 



order includes a group of extraordinary flying Eeptiles, all 

 belonging to the Mesozoic epoch, and exhibiting in many 

 respects a very extraordinary combination of characters. 



Fig. 571. — A, Skiall of Dicynodon lacerticeps, showing the maxillary tusk ; b, Skull of 

 Oudenodon Bainii. From the Trias of South Africa. (After Owen.) 



The most familiar members of tlie order are the so-called 

 " Pterodactyles," and the following are the characters of the 

 order : — 



Ho exoskeleton is known to have existed. The dorsal vertebras 

 are procodous, and the anterior trunk-ribs are double-headed. 

 There is a broad sternum vnth a median ridge or keel, and 

 ossified sternal ribs. The jaws loere generally armed tvith teeth, 

 and these ivere im,planted in distinet sockets. In some forms 

 {Rhamphorhynchus) 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 Ptcranodon, 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 oecurs in the sclerotic coat of the eye. 



