INTRODUCTION. 



The class of Birds (Avcs), as represented in the present age of the world, 

 is composed of very many species, closely related among themselves and 

 distinguished by numerous characters common to all. For the purposes of 

 the present work it is hardly necessary to attempt the definition of what 

 constitutes a bird, the veriest tyro being able to decide as to the fact in 

 regard to any North American animal. Nevertheless, for the sake of greater 

 completeness, we may say that, compared with other classes,^ Birds are 

 abranchiate vertebrates, with a brain filling the cranial cavity, the cerebral 

 portion of which is moderately well developed, the corpora striata connected 

 by a small anterior commissure (no corpus callosum developed), prosen- 

 cephalic hemispheres large, the optic lobes lateral, the cerebellum trans- 

 versely multifissured ; the lungs and heart not separated by a diaphragm 

 from the abdominal viscera ; aortic arch single (the right only being devel- 

 oped) ; blood, with nucleated red corpuscles, undergoing a complete circula- 

 tion, being received and transmitted liy the right half of the quadrilocular 

 heart to the lungs for aeration (and thus warmed), and afterwards returned 

 by the other half through the system (there being no communication be- 

 tween the arterial and venous portions) ; skull with a single median convex 

 condyle, chiefly on the basi-occipital (with the sutures for the most part early 

 obliterated) ; the lower jaw with its rami ossifying from several points, con- 

 nected with the skull by the intervention of a quadrate bone (homologous 

 with the malleus) ; pelvis with ilia prolonged in front of the acetabulum, ischia 

 and pubes nearly parallel wdth each other, and the ischia usually separated : 

 anterior and posterior members much differentiated ; the former modified for 

 flight, with the humerus nearly parallel with the axis of the body and con- 

 cealed in the muscles, the radius and ulna distinct, with two persistent carpal 

 liones, and two to four digits ; the legs with the bones peculiarly combined, 

 (I) the proximal tarsal bones coalescing with the adjoining tibia, and (2) 

 the distal tarsal coalescing with three (second, third, and fourth) meta- 

 tarsals (the first metatarsal being free), and forming the so-called tarso- 

 metatarsus ; dermal appendages developed as feathers : oviparous, the eggs 

 being fertilized within the body, excluded with an oval, calcareous shell, and 



1 We are indebted to Professor Theodore N. Gill for the present account of the characteristics 

 of the class of Birds as distinguished from other vertebrates, pages xi - xv. 



