KEPT I LI A. 447 



Among the enormous number of extinct Reptilia we may 

 here merely notice a few. 



The Pterodactyles (prditx Fterosatiria) were winged rep- 

 tiles, with the wing formed of a membrane stretched from 

 the enormously elongated fifth digit. They had a skull 

 somewhat like that of a bird, but with teeth. 



The Ichthyosauria were large fish-lizards with long tail 

 and the limbs modified into flippers. The skull had a 

 rostrum like that of the porpoise. Ichthyosaurus is a 

 common example. 



The Dinosauria were large terrestrial reptiles, some of 

 which show structural features resembling birds. Iguanodo7t 

 is perhaps the be'st known. 



Lastly, the Theromorpha appear to be reptiles showing 

 remarkable resemblances to mammals, especially in the 

 heterodont dentition; some of this group also point to 

 relationships with fossil Amphibia. 



Class V. — Aves. 



Birds are closely allied to the reptiles in their structure, 

 but they are so completely adapted for an serial habit that 

 there is no difficulty in at once distinguishing them. They 

 resemble the reptiles, especially in their skeletal structure, 

 the similar bones of the skull, the suspension of the 

 mandible by the quadrate, the many elements of the 

 mandible, the single ear-bone or columella and the absence 

 of epiphyses. In addition, they have the same oviparous 

 habit, with meroblastic segmentation, and the same foetal 

 membranes. These and other similarities are sometimes em- 

 phasised by the grouping of the two classes together under 

 the head of Sauropsida, 



On the other hand, the birds show the following adapta- 

 tions to an aerial habit. The fore-limbs are not used for 

 terrestrial locomotion, as in reptiles, but are formed into 

 wings, the method of formation involving the entire loss 

 of the two postaxial digits and a great reduction of the 

 preaxial. Probably at first each digit had its separate tuft 

 of flight feathers or alula^ but in all modern birds the alula 

 of the first digit alone remains, those of the second and 

 third combining with the flight-feathers of the ulna to form 



