STERNA OP THE GCLLS. 397 



walking on the shore, to the petrels, which are more 

 exclusively seaward, and walk little : but there are 

 so many modifications of the sternal apparatus, that 

 no single bird can be selected as an average type of 

 the whole. We shall therefore select two speci- 

 mens : the laughing gull as nearly an average of the 

 more landward subdivision, and the puffin petrel as 

 nearly an average of the more seaward. 



The sterna of the skuas, gulls, and terns, are in 

 their general form intermediate between those of the 

 entire footed birds and of the waders. They are 

 rather longer and not quite so broad in proportion as 

 those of the former ; but broader and less elongated 

 than those of the latter. The keel also, though well 

 developed, is not so high in proportion to the width 

 of the sternum. From this structure it follows that 

 the motion of their wings, though more powerful, 

 must be slower than that of the waders; and the 

 wing itself is longer in its bones and its feathers, as 

 well as in those muscles which put it in motion. 

 This structure accords with the general habit, which 

 is that of skimming about with smooth and mode- 

 rately swift wing, and culling their food from the 

 crests of the waves, or from substances cast up by the 

 waters and left upon the strand. The keel of the 

 sternum is deeply concave in front, with the angle 

 much rounded ; and the posterior part, which is 

 nearly square at the end, is divided more or less 

 deeply by four notches, the two middle ones of which 

 are larger in some of the species, and the two lateral 

 in some of the others. The posterior, or carrying 

 part, of the sternum is rather broader than the middle 

 part to which the ribs, which are six or seven, are 

 attached ; and the lateral processes extend on the 

 flanks without any bending inwards. The coracoids 



