BEWICK'S SWAN. 



205 



In anatomical structure this new species differs much 

 more decidedly from the Hooper than in its external cha- 

 racters. The principal and most obvious difference is in 



the trachea, which forms one of the best distinctions in the 

 separation of nearly-allied species throughout this nume- 

 rous family. The tube of the windpipe is of equal dia- 

 meter throughout, and descending in front of the neck 

 enters the keel of the sternum, which is hollow, as in the 

 Hooper, traversing its whole length. Having arrived at 

 the end of the keel, the tube then gradually inclining up- 

 wards and outwards, passes into a cavity in the sternum 

 destined to receive it, caused by a separation of the parallel 

 horizontal plates of bone, forming the posterior flattened 

 portion of the breast-bone, and producing a convex pro- 

 tuberance on the inner surface. The tube also changing 

 its direction from vertical to horizontal, and reaching with- 

 in half an inch of the posterior edge, is reflected back after 

 making a considerable curve, till it once more reaches the 

 keel, again traversing which, in a line immediately over 

 the first portion of the tube, it passes out under the arch 

 of the merrythought ; where, turning upwards, and after- 

 wards backwards, it enters the body of the bird to be at- 



