870 SKULL 
size of acommon Gull, Larus canus, and presenting, irrespective of sex, 
two very distinct phases of plumage, one almost wholly sooty-brown, 
the other particoloured—dark above and white on the breast, the 
sides of the neck being of a glossy straw-colour, and the lower part 
of the neck and the sides of the body barred with brown; but a 
singular feature in the adults of this species is that the two median 
tail-feathers, which are elongated, have their shaft twisted towards 
the tip, so that in flight the lower surfaces of their webs are pressed 
together vertically, giving the bird the appearance of having a disk 
attached to its tail. The second and third species so closely 
resemble each other, except in size, that their distinctness was for 
many years unperceived, and in consequence their nomenclature is 
an almost bewildering puzzle. Mr. Saunders (tom. cit. p. 322) thinks 
that the larger of them, which is about the size of a Black-headed 
Gull, should stand as S. crepidatus, and the smaller as S. parasiticus, 
though the latter name has been generally used for the larger when 
that is not termed, as it often is, S. richardsoni—a name that 
correctly applies only to whole-coloured examples, for this species 
too is dimorphic. Even its proper English name? is disputable, 
but it has been frequently called the Arctic Gull or Arctic Skua, 
and it is by far the commonest of the genus in Britain, and perhaps 
throughout the northern hemisphere. It breeds abundantly on 
many of the Scottish Islands,? and in most countries lying to the 
northward. The nest is generally in long heather, and contains 
two eggs of a dark olive colour, suffused with still darker brown 
patches. Birds of either phase of plumage pair indiscriminately, 
and the young shew by their earliest feathers whether they will 
prove whole or particoloured ; but in the immature dress of the last 
the upper surface is barred with pale reddish-brown. The smallest 
species, commonly known in English as the Long-tailed or Buffon’s 
Skua, rarely exhibits the remarkable DIMORPHISM (p. 149) to which 
the two preceding are subject, but one instance (Ibis, 1865, p. 217) 
apparently being on record. It breeds abundantly in some seasons 
on the fells of Lapland, its appearance depending chiefly on the 
presence of Lemmings, on which it mainly preys. All these three 
species occasionally visit the southern coasts of Europe in large 
flocks, but their visitations are highly irregular. 
SKULL, the comprehensive word for all the bones of the head, 
which may be conveniently grouped as those forming the CRANIUM 
1 It is the Fasceppar or Fasgadair of the Hebrides, the SuHoor of the Shet- 
lands, and the ScouTI-ALLEN of the fishermen of Orkney and on the east coast 
of Scotland. 
2 Pennant was the first to discover that it bred in the British Islands, by 
finding it on the Ist of July 1772 on Jura, which, thanks to the protection 
accorded to it, it still inhabits, and this must be the most southerly point in its 
breeding-range. 
