Mr. Yarket^l on the Trachea of Birds. 387 



the latter diminishing from six inches in the tirst to four and 

 a half in the Tufted Duck, three inches in the Long-tailed, 

 and but two in the Golden Eye. The ribs of the birds of this 

 division are elongated ; the keel of the breast-bone gradually 

 decreases in depth ; the position of the wings is more forward, 

 the legs placed further back. The trachea? of these Ducks 

 are particularly distinguished from those of the others by the 

 enlaroement at the bottom of the tube being covered with a 

 delicate membrane, supported by slender portions of bone ; the 

 trachea of the Red-crested Duck (Tab. XV. e.) is an example 

 of this form, and may be considered the type of this divi- 

 sion. As the Egyptian Goose has in this arrangement been 

 considered the link between the Geese and the first division 

 of the true Ducks, from its possessing, with the characters 

 of the former, the bony enlargement of the trachea com- 

 mon to the latter ; and the Velvet Duck, for similar reasons, 

 supplying the link between the two divisions of true Ducks, 

 possessing, among other characters, an altered form of the 

 bony enlargement of the trachea of one, with the lobated 

 toe of the other; so the Golden Eye, the last of the series, 

 appears to complete the arrangement, by exhibiting some of 

 the characters found in the Mergansers, which are next in suc- 

 cession. The tirst point of similarity is in the elongated feathers 

 on the top of the head, forming a crest ; they agree also in tlie 

 shape of the sternum, and a particular extension of its posterior 

 edge, becoming an ensiform process, the appearance of which 

 in the Goosander is represented in Tab. XV. marked (g): 

 and this extension of the edge of the breast-bone prevails in 

 the species of the genera Colymbus, Alca, and Uria ; and with 

 the elongation of the ribs observable in all good salt-water 

 Divers, seems intended as a protection to the important viscera 

 of the abdomen, and enables them to resist pressure when below 



the 



