588 COLLEGE ZOOLOGY 



shaped, highly vascular, pigmented structure called the pecten, 

 which is suspended in the vitreous humor. The function of the 

 pecten is uncertain; it may have some connection with the nutri- 

 tion of the eyeball, or with the process of accommodation. The 

 latter process is remarkably well developed in birds, since their 

 eyes are equally adapted both for far and near vision, and a bird 

 can fly rapidly among the branches of a tree without striking a 

 branch, or can swoop down to the ground from a great height in the 

 air, changing from far-sighted to near-sighted vision in an instant, 



2. A Brief Classification of Birds 



The birds form a more homogeneous class of vertebrates than 

 the reptiles and cannot be separated into a few well-defined 

 groups. There are comparatively few fossil birds known to 

 man; in fact, only one subclass, containing a single genus, and 

 four orders, are not represented by living forms. The structural 

 differences that distinguish the orders, families, genera, and 

 species are, for the most part, so slight as to make it 

 impossible to state them in a brief and clear manner. 



More than twelve thousand species of birds have been de- 

 scribed, and no two authorities agree as to their classification. 

 The following arrangement is adopted from Knowlton's Birds 

 of the World. 



Class Aves. Birds. — Warm-blooded vertebrates with 

 feathers; usually with fore limbs adapted for flight; the adults 

 of existing species without teeth. 



Subclass I. Archleornithes. — Ancient, reptile-like, fossil 

 birds. Only three specimens of the single genus Archaopteryx 

 are known. 



Subclass II. Neornithes. — Recent Birds. — There are 

 four orders containing only extinct forms, and seventeen orders 

 containing living representatives. 



Order i. Hesperornithiformes. — Fossil, toothed-birds from 

 America, with teeth set in a groove. Example: Hesperornis 

 (Fig. 478). 



