530 DIVING BIRDS. 



The preceding membei-s of the family collectively constitute 

 Wilson s PetreL ^ subfamily characterised by the presence of at leaat thirteen 

 secondary quills, by the metatarsus being covered with small hexagonal plates, 

 by the sharp and curved claws, and by the leg-bones being shorter than the 

 wino-. On the other hand, the small Wilson's petrel, together with some allied 

 species, forms a second subfamily distinguished by the presence of only ten 

 secondaries, by the metatarsus being either booted or covered in front with large 

 oblique scutes, by the flat and bixwid claws, and by the leg -bones exceeding 

 the wings in length. In all the group the aperture of the straight nasal tubes 

 is sino-le and circular. Wilson's peti-el (Oceanites oceanicus), which is somewhat 

 larger than the storm-petrel, inhabits the Atlantic Ocean and Australian seas; 

 while the other members of the genus are exclusively southern. The general 

 colour of its plumage is dusky, with the quills and tail-feathers black. As regards 

 their muscles, these petrels are highly specialised, and in the boot-like plates 

 covering the metatarsus, they differ from all other water-birds. The white-bellied 

 petrel (Cymodroma grallaria), of the tropical seas, has an even tail, and the 

 metatarsus about twice the length of the third toe, exclusive of the claw. The 

 plumage is parti-coloured. 



The Diving Like the albatrosses, all the petrels hitherto mentioned are 



Petrel. essentially flying and swimming-birds, which dive but little. There 

 is, however, a remarkable aberrant petrel inhabiting the Straits of Magellan which 

 differs from all the other members of the order in its short wings and diving 

 habits; while it is further distinguished by the nasal tubes being vertical and 

 opening superiorly ; the first toe being also absent. This bird (Pelecanoidea 

 urxTuitrix), which many ornithologists regard as the representative of a distinct 

 family, is, indeed, in habits and appearance so like an auk, that, as Darwin 

 remarks, when seen from a distance, either on the wing, or diving and swimming, 

 it would undoubtedly be mistaken for one of these birds. Nevertheless, both in 

 structure and plumage, it is essentially a petrel ; and we must accordingly regard 

 its auk-like appearance and habits as special modifications for a peculiar mode of 

 life. These birds, according to Moseley, may be seen in calm weather in Royal 

 Sound floating in immense numbers on the water, the flocks sometimes extending 

 over acres, and causing the sea to appear literally black. They dive with extreme 

 rapidity, and when disturbed, rise and flutter a short distance along the surface, 

 after which they again drop and dive. 



The Diving Birds. 



Order Pygopodes. 



This group admits of much less concise definition than the preceding, unless 

 indeed we follow the example of many modem ornithologists in removing 

 from it the auks. It has been proposed to transfer the latter to the gulls, 

 with which they agree in having cup-and-ball articulations to the vertebrae 

 of the back; but since it is certain that all the primitive birds had cup-like 

 articular surfaces to their vertebrae, there seems no reason why the cup-and-ball 



