286 NATATORES. CYGNUS. SWAN. 



longer, is more slender than in the Whistling Swan. In in- 

 ternal conformation, particularly as regards the trachea and 

 sternum, the differences are very striking. In the new spe- 

 cies, the cavity of the sternum, instead of being restricted to 

 a depth of three inches, or three and a quarter (as I have be- 

 fore stated it to be in the Whistling Swan), is frequently 

 found to extend to five and a half or six inches, and, after 

 reaching the posterior extremity of the keel, to occupy in ad- 

 dition a portion of the lateral part of the sternum ; and here 

 the trachea, instead of making a vertical Jteocure^ as in the 

 preceding species, is forced to take a horizontal bend, and to 

 form a loop, as it were, in the excavated part of the sternum. 

 The keel of C. Bewickii, also, is not so deep as that of the 

 other, and consequently the two portions of the trachea with- 

 in the arched cavity are brought closer together. Other 

 marked differences are observable in the trachea of the new 

 species after its egress from the cavity of the sternum, as 

 compared with the corresponding parts of the other. In it 

 the trachea, after describing the bend, on its egress from the 

 keel, enters into the cavity of the thorax for upwards of two 

 inches, and is then attached by the lower larynx (or bone of 

 divarication) to the bronchial tubes ; in the Whistling Swan, 

 on the contrary (as I have previously stated), no portion of 

 the trachea enters within the thorax, but the lower larynx 

 reaches just as far as the anterior ridge of the sternum, up- 

 on which it rests obliquely. The dimensions of the trachea 

 where it joins the lower larynx, and that part itself, are very 

 dissimilar in the two species, the present one having the dia- 

 meter as large at that junction as at any other part of the 

 tube, and the larynx short, broad, and but slightly com- 

 pressed. In the size and form of the bronchi, also, there is 

 considerable difference, their length and diameter being 

 scarcely equal by one-half to those of the Whistling Swan, 

 and the rings of which the tubes are composed being of a 

 different shape. In addition, the sternum of C. Bewickii is 

 much broader in comparison to the size of the bird, and the 



