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the length of the oesophagus and proventriculus, and in the 

 dimensions of the stonnach, vitelline remnant, and trachea. The 

 tympanum of the Clangula sends two bony plates up on the 

 front and back of the trachea, rendering it perfectly inflexible to 

 the distance of an inch ; this does not exist at all in the Mergus, 

 and only to the extent of a quarter of an inch in the hybrid. 

 The tracheal dilatation is about an inch long in the Mergus, two 

 inches in the Clangula, one and a half inches in the hybrid. In 

 the Mergus, it becomes flat when stretched, and dqes not become 

 elongated more than any other part of the trachea ; in the Clan- 

 gula, owing to the oblique arrangement of the rings, its long and 

 transverse diameter are both increased ; in the hybrid, its poste- 

 rior surface is somewhat flat, and it is not elongated more than 

 any other part of the trachea. The posterior surface of this 

 part, in the Mergus, is membranous, and soft ; in the Clangula 

 and hybrid, the rings of the dilaiation are firmer throughout their 

 circumference than those of the trachea elsewhere. Rings of 

 the dilatation in the Mergus, 15, very much flattened, and extend- 

 ing only half around the circumference ; in the Clangula, 19, 

 firm, almost bony, complete ; in the hybrid, 14, not so firm, and 

 somewhat flattened posteriorly. The last half inch of the trachea 

 in the Mergus, has a pent-house ridge, forming an acute angle 

 on its anterior aspect, which is wanting in the Clangula, and 

 present, in a slight degree, in the hybrid. Left bronchus in the 

 Clangula, y\ of an inch across ; in the hybrid, -^^ ; in the Mer- 

 gus, y\ ; the left being the larger in the two former, and the 

 right in the latter. The middle and outer toe of the hybrid are 

 the longest ; the Mergus has the middle the longest, and the Clan- 

 gula the outer. It resembles the Mergus in the markings of the 

 secondaries of the wings, and those on the flank ; in the com- 

 mencing appearance of the dark half collar, between the neck 

 and shoulder, extending round toward the median line of the 

 breast ; in the descent of the dark feathers low down upon the 

 neck ; in the slight appearance of the white coloration of the 

 wing coverts ; in having eighteen, instead of sixteen tail feathers, 

 as in the Clangula ; in the size and proportions of the scapula; 

 and in the size and arrangement of the scales of the tarsus. It 

 resembles the Clangula in the size and proportions of the sternum, 



