Hi LECTURES ON EVOLUTION 111 



There is another series of extinct reptiles 

 which may be said to be intercalary between 

 reptiles and birds, in so far as they combine some 

 of the characters of both these groups ; and which, 

 as they possessed the power of flight, may seem, 

 at first sight, to be nearer representatives of 

 the forms by which the transition from the 

 reptile to the bird was effected, than the 

 Ornithoscelida. 



These are the Pterosauria, or Pterodactyles, the 

 remains of which are met with throughout the 

 series of Mesozoic rocks, from the lias to the chalk, 

 and some of which attained a great size, their 

 wings having a span of eighteen or twenty feet. 

 These animals, in the form and proportions of the 

 head and neck relatively to the body, and in the fact 

 that the ends of the jaws were often, if not always, 

 more or less extensively ensheathed in horny beaks, 

 remind us of birds. Moreover, their bones con- 

 tained air cavities, rendering them specifically 

 lighter, as is the case in most birds. The breast- 

 bone was large and keeled, as in most birds and in 

 bats, and the shoulder girdle is strikingly similar 

 to that of ordinary birds. But, it seems to me, 

 that the special resemblance of pterodactyles to 

 birds ends here, unless I may add the entire 

 absence of teeth which characterises the great 

 pterodactyles (Pteranodori) discovered by Professor 

 Marsh. All other known pterodactyles have teeth 

 lodged in sockets. In the vertebral column and 



