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CHAPTER LVI. 



CLASS AVES. 



General Structure. 



The fifth class of the Vertebrata is that of Aves, or Birds, which, 

 as we have already mentioned under the description of the class 

 Reptilia, presents a number of characters in common with the 

 latter. Birds, indeed, as Professor Huxley remarks, are animals 

 so similar to Reptiles in all the most essential features of their 

 organisation, that they may be said to be merely an extremely 

 modified and aberrant Reptilian type. Their differentiation is, 

 however, so great as to indicate without doubt their right to form 

 a distinct class. It will be unnecessary to recapitulate the char- 

 acters common to Birds and Reptiles — together constituting the 

 province Sauropsida — and we may accordingly proceed to note 

 the distinctive features of the former class. It may be well, how- 

 ever, to observe before proceeding further that, according to the 

 arrangement proposed by Professor A. Newton, Birds are divided 

 into three primary divisions or orders, respectively known as 

 Saururae, Ratitae, and Carinatae ; the fossil representatives of which 

 will be noticed in the next chapter. 



In the first place, all Birds, so far as can be ascertained, were 

 provided with the peculiar epidermal covering known as feathers, 

 which are totally unknown among the Reptiles ; while ossifications 

 in the dermis are extremely rare, and never take the form of bony 

 scutes. No Bird, again, has proccelous vertebrae ; while in all ex- 

 isting forms the centra of the cervicals have cylindroidal, saddle- 

 shaped, articular surfaces, although these are amphiccelous in certain 

 Mesozoic forms. In no cases are there sacral ribs for attachment 

 of the ilia in the proper sacral vertebrae. The sternum has no 

 backwardly-produced median processes for the ribs ; all of which 

 are attached superiorly to its lateral borders. If an interclavicle 

 ever exists, it is fused with the clavicles into a compound bone 



