33 CLASSIFICATION OF VERTEBRATES. 



spheres, the widely separated optic lobes, and the presence of a 

 lateral projection (flocculus) from either side of the cerebellum. 



In the PTERODACTYLID^: teeth were present; in the PTERANODON- 

 TID,E they were lacking. The pterodactyls proper had a short tail ; in 

 others of the family it was longer. Pteranodon has been found only in 

 the middle cretaceous of Kansas, Dimorphodou from the lower lias of 

 Dorsetshire. 



SUB-CLASS II. AVES. 



Sauropsida with the body covered with feathers ; anterior 

 appendages modified for flight ; warm blooded ; heart completely 

 divided ; only one (right) pertistent aortic arch ; oviparous. 



The group of birds is strongly marked off from all other ver- 

 tebrates by the feathers. No other animals, recent or extinct, 

 are known which had this protective envelope, and no birds lack 

 them. The structure and development of these characteristi- 

 cally avian features have been described (p. 94), but their 

 arrangement is of considerable importance in classification. 



Except in a few birds the feathers are not distributed evenly 

 over the body, but are in distinct tracts or pterylae, these being 

 separated by spaces (apteria) with no feathers or with only down- 

 feathers. The feather tracts of the wings bear the feathers of 

 flight which, according to the part on which they are supported, 

 receive different names. Those attached to the hand are the 

 primaries, to the fore arm secondaries, the three proximal of the 

 fore arm feathers being the tertiaries. Primaries, secondaries, 

 :and tertiaries together are called the remiges. These remiges 

 .are overlaid above and below by shorter feathers, the upper or 

 under wing coverts. The principal tail-feathers are the rectrices, 

 :and these are similarly overlaid by the tail coverts. The feath- 

 ers attached to the first digit form the ala spuria or alula. Some 

 birds are completely naked when hatched, and are called gymno- 

 paedes. Others (dasypaedes) are covered with down upon their 

 escape from the egg, while a few (pteropaedes) have the contour- 

 feathers developed before hatching. All of the gymnopaedes 

 and some of the dasypaedes are fed by the adult, and are conse- 

 quently called altrices ; but most of the latter group can run 

 about at once (praecoces). 



