STERNA OF THE PETRELS. 399 



petrels. The preceding figures of the sternum of the 

 laughing gull, and more especially the lower one which 

 represents the profile, will illustrate this structure. 



The sternal apparatus of the petrels is much better 

 formed both for flight and for resistance of pressure 

 against the front. It is short and broad, rather nar- 

 rower in the middle than towards the extremities, 

 very concave on the upper surface, and consequently 

 convex on the under. The keel much produced, 

 extending with some elevation to the posterior ex- 

 tremity of the sternum, but much more elevated in 

 front, convex in its under outline, and concave in the 

 front one ; the angle much produced, rounded, and 

 supporting the clavicle, which is enlarged at the 

 junction of its branches. The lateral processes of 

 the sternum are well produced, the grooves deep, and 

 the coracoids firmly attached by large bases. The 

 sides concave upwards, carrying six ribs. The poste- 

 rior edges differing in different genera, being entire 

 in the storm-petrels, having a large notch on each 

 side in the albatross, and two small ones on each side 

 in most of the others. The coracoids are strong in 

 their whole length, but remarkable for the extent of 

 their bases and the largeness of their heads, from the 

 first of which structures they form a perfect arch in 

 the lateral directions, even without the assistance of 

 the clavicle. But the clavicle is also strong, and 

 capable from its curvature of offering a powerful re- 

 sistance to any- strain, whether lateral or from the 

 front. The scapular is slender, very slightly curved 

 . downwards, and continued in the same curve wuth 

 the upper part of the clavicle. This is indeed one 

 of the firmest shoulders which occurs in the whole 

 class ; and, as will be seen from the profile in the 

 following figure, the keel of the sternum, the coracoid, 



