320 



ANATID^. 



from the Whooper than in its external characters. The 

 most obvious difference is in the trachea, which forms one of 

 the best distinctions in the separation of nearly-allied species 

 throughout this numerous family. The tube of the wind- 

 pipe is of equal diameter throughout, and descending in 

 front of the neck enters the keel of the sternum, which is 

 hollow, as in the "Whooper, traversing its whole length. 

 Having arrived at the end of the keel, the tube then, 

 gradually inclining upwards 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 pro- 

 ducing a convex protuberance on the inner surface. The 



tube also changing its direction from vertical to horizontal, 

 and reaching within 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 afterwards backwards, it enters the body of 

 the bird to be attached to the lungs in the usual manner. 

 This is the state of development in the oldest bird the 

 Author met with. The degree next in order, or younger, 

 differs in having the horizontal loop of the trachea confined 

 to one side only of the cavity in the sternum, both sides of 



