AVES. 



BIRDS are warm-blooded vertebrate animals, oviparous, and covered 

 with feathers. The anterior limbs are modified into wings. The 

 skull articulates with the vertebral column by a single occipital 

 condyle, and the jaw is connected with the skull by the interven- 

 tion of a quadrate bone. The heart consists of four chambers, 

 two auricles and two ventricles, and the right and left sides are 

 completely separated from one another. There is only one aortic 

 arch, the right. 



It is usual to divide all living birds into two great subclasses, 

 which are diagnosed from each other by the shape of the sternum. 

 In one subclass, the Carinatce, the sternum is typically provided 

 with a keel ; in the other, the Ratitce, the keel of the sternum is 

 absent. 



Although this primary division of birds is convenient in many 

 ways, yet there are exceptions to its application which render a 

 classification based on the shape of the sternum of doubtful utility. 

 Some birds which from other points of view are undoubtedly 

 Carinatce have the keel of the sternum little, if at all, developed. 



I prefer therefore to divide birds at once into groups which I 

 shall term Orders, and in doing so I shall avail myself of the recent 

 studies of Mr. Seebohm. This gentleman, partly by independent 

 osteological investigations of his own, and partly by utilizing the 

 discoveries of other workers in the same or similar fields, has, with- 

 out disturbing the usually accepted classification of birds to any 

 great degree, arrived at an arrangement which possesses the merit 

 of being precise and clear, so far as the materials at his disposal 

 have enabled him to be so. He has, moreover, diagnosed the 

 different Orders by characters which the least skilful can easily 

 investigate and discover for themselves. 



Mr. Seebohm divides birds into several large groups which he 

 terms Orders, and these again into suborders which are equal to 

 the groups which I, in accordance with the usual practice, prefer to 

 call Orders. I do not propose to treat of the distinctions between 

 the different Orders here, but to deal with them at the end of this 

 work, as I gather from Mr. Seebohm that he contemplates a revi- 

 sion of them. The period of two years which, moreover, must 

 elapse before the present work is completed cannot fail to be pro- 

 ductive of much additional information and improvement with 

 respect to the classification of birds. 



YOL. I. B 



