104 STRUCTURE AND COMPARISON. 



They advance into the water up to their bellies, and if it should 

 suddenly deepen they keep right on by swimming, not at all 

 incommoded by the loss of their foothold." 



Here is a shape you would hardly have dared invent, a bird 

 with the under mandible much longer than the upper and as 

 thin as a knife-edge. There are but three such bills in the 

 world, and all belong to different species of skimmers. 



FIG. 31. HEAD OF BLACK SKIMMER. 



"This bird," says Wilson, "is formed for skimming, while 

 on the wing, the surface of the sea for its food, which consists 

 of small fish, shrimps, young fry, etc., whose usual haunts are 

 near the shore and towards the surface. That the lower man- 

 dible, when dipped into and cleaving the water, might not retard 

 the bird's way, it is thinned and sharpened like the blade of a 

 knife ; the upper mandible, being at such times elevated above 

 the water, is curtailed in its length, as being less necessary, 

 but tapering gradually to a point, that, on shutting, it may 

 offer less opposition. To prevent inconvenience from the 

 rushing of the water, the mouth is confined to the mere open- 

 ing of the gullet, which indeed prevents mastication taking 

 place there; but the stomach, or gizzard, to which this business 

 is solely allotted, is of uncommon hardness, strength, and 



