1882. } FROM PERU AND CHILI, 523 
upper and under sides are dark smoke-colour, with merely a 
thin outside margin of a lighter tint; the under wing-coverts 
are dusky; the tips of the secondaries show far less white than 
in the northern bird; whilst the upper primaries can hardly be 
said to show any trace of lighter colour even in this Coquimbo speci- 
men, which is a fully adult and a freshly-moulted example. The dis- 
tinctions admittedly rest upon the respective preponderances of light 
and dark; and in the immature northern bird, such as I consider 
the Central-American example to which reference has been made, 
the tail is not so white as in the adult; nevertheless the two forms 
can be separated at a glance at any age. It is substantially correct 
to define the northern A. nigra as a white-tailed, and the southern 
Rt. melanura as a black-tailed bird. In the winter plumage both 
species show a more or less defined white collar. 
There is a break in the chain of evidence respectiug the inter- 
tropical range of these two species. Through the kindness of my 
friend Mr. E. Hargitt, I have specimens of the southern form, R. me- 
lanura, from as far north as Berbice, British Guiana, about lat. 6° 
N.; and it probably ascends the Orinoco, and passes along the coast 
of Venezuela. In this case the northern range of R. melanura 
in the Atlantic would come very close to the territory of the northern 
R. nigra, which is common in Florida, whence I possess examples. 
Under these circumstances the wonder is that the distinctions 
between the northern and southern forms should be so marked as 
they are. 
XemMa FuRCATUM (Neboux). (Plate XXXIV.) 
Larus furcatus, Neboux, Voy. ‘ Vénus,’ Atlas, pl. x. (1846), deser. 
Rev. Zool. 1840, p. 290. 
Creagrus furcatus, Bp. Naum. 1854, p. 213; Salvin, Tr. Z. S. 
ix. p. 506. 
Xema furcatum (Neboux), Saunders, P. Z. S. 1878, p. 210. 
[No. 9, Paracas Bay, Peru, Oct. 1881. Eyes brown. | 
The third known example of this rarest of Gulls, the history of 
which may here be briefly recapitulated. The Paris Museum possesses 
one, in somewhat immature plumage, said to have been obtained by 
Dr. Neboux, of the French frigate ‘Vénus,’ at Monterey, California, 
in the month of November. The British Museum has an adult in 
full breeding-plumage, obtained during the voyage of H.M.SS. 
‘Herald’ and ‘ Pandora,’ at Dalrymple rock, Chatham Island, Gala- 
pagos group, nearly on the equator, between the 11th and 16th 
January. It is a medium-sized Gull, with long wings (16 inches), 
a dark slate-coloured hood, and a forked tail; indeed were it not 
that the hood is separated from the base of the hill by a band of 
white feathers, and that there is no black neck-ring at the base of 
the hood, Xema furcatum might be described as a gigantic Sabine’s 
Gull. In the young, now figured, the resemblance to the young of 
Xema sabinii is very marked. The entire head is white, with dark 
markings in front of and surrounding the eyes, and a brown auri- 
cular patch as in most of the immature hooded Gulls; neck and 
