594 REPTILES. 



Megalosaurus is a good type. The pelvis has a Crocodilian aspect, 

 for the pubes slope forwards instead of backwards as in Birds and 

 Iguanodon, &c. The limbs were furnished with powerful claws, and 

 the teeth show much specialisation. Stcgosaurus was furnished with 

 heavy armour of plates and spines. Triceratops had three horns on its 

 enormous head. The point of greatest interest about the Dinosaurs 

 is the resemblance to Birds. This was first insisted on by Huxley, 

 and since then it has been generally held that Birds have diverged 

 from a Dinosaur stock. It is, however, fair to notice that by some 

 these resemblances have been declared to be unimportant, while the 

 points of resemblance between Birds and the next order of Reptiles 

 are much dwelt upon. 



Ornithosauria. Flying Reptiles, represented from the lower Jurassic 

 to the Upper Chalk, exhibiting many points of resemblance to Carinate 

 Birds, but still distinctly Reptilian in type. An expansion of the skin 

 seems to have been stretched on the much elongated outermost ringer, 

 and to have extended backwards to the hind legs and the tail. The 

 long bones contained air-sacs as in many Birds. The sternum is keeled, 

 and teeth are often present on both jaws. Some are said to have had 

 an expanse of wing of nearly twenty-five feet, but others were no 

 larger than sparrows. It is a question how far the resemblances ot 

 these forms to Birds are a consequence of similar habits, and how far 

 they can be regarded as indicating true affinities. 



Examples : Pterodactylus, Rhamphorhynchus, Pteranodon . 



Relationships. 



While it is still rash to venture on general conclusions, 

 this much seems clear that the Reptiles, in their widest 

 sense, form a central assemblage among Vertebrates. As 

 we have noted above, some of the extinct forms exhibit 

 affinities with Amphibians, others with Birds, others again 

 with Mammals. Though we cannot with certainty point to 

 any of the extinct types as directly ancestral to either Birds 

 or Mammals, it seems likely that the ancestors of both 

 were derived from the plastic Saurian stock. 



