REPTILIA. 447 
Among the enormous number of extinct Repiz/ia we may 
here merely notice a few. 
The Pterodactyles (order Prerosauria) 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 Lchthyosauria were large fish-lizards with long tail 
and the limbs modified into flippers. The skull had a 
rostrum like that of the porpoise.  Jchthyosaurus is a 
common example. 
The Dinosauria were large terrestrial reptiles, some of 
which show structural features resembling birds. Lewanodon 
is perhaps the best known. 
Lastly, the Zheromorpha appear to be reptiles showing 
remarkable resemblances to mammals, especially in the 
.heterodont dentition; some of this group also point to 
relationships with fossil Amphrbza. 
Crass V.—AVES, 
Birds are closely allied to the reptiles in their structure, 
but they are so completely adapted for an erial 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 fcetal 
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 serial 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 a/z/a, 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 
