530 DIVING BIRDS. 
The preceding members of the family collectively constitute 
a subfamily characterised by the presence of at least 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 
wing. 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 seutes, by the flat and broad claws, and by the leg-bones exceeding 
the wings in length. In all the group the aperture of the straight nasal tubes 
is single and circular. Wilson’s petrel (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 (Pelecanoides 
wrinatrix), 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. 
Wilson’s Petrel. 
THE Divina BIRDS. 
Order PYGOPODES. 
This group admits of much less concise definition than the preceding, unless 
indeed we follow the example of many modern 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 vertebra, there seems no reason why the cup-and-ball 
