INTRODUCTIOISr 



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

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

 distinguished by numerous characters comraon to all. For tlie 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 iu 

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

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

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

 portion of wliich is moderately well de\'eloped, the corpora striata connected 

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

 cephalic hemisi)heres 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 by the right half of the quadrilocular 

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

 by the otlier half througli the system (there being no communication be- 

 tween the arterial and \'enous portions) ; skull with a single median convex 

 condyle, chietly on the Ijasi-occipital (witli tlie sutures for the most part early 

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

 nected with the skull by tlie inter\'ention of a (juadrate bone (homologous 

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

 and pubes nearly parallel with each other, and the ischia usually sepai'ated : 

 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 

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

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

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

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

 metatarsus ; dermal ai)pendages develo]ied as feathers : oviparous, the eggs 

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



' AVe are iudpbtctl to Profus-sor Theodore N. (iill for tlie present aceoiuit of tlie eliaracteiistics 

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



