Diacope marginata, _Cuv. et Val._ Hist. des Poiss. tom. ii. p. 320.
FORM.—Greatest depth of the body and length of the head equal, each
being not quite one-third of the entire length. Nape somewhat
elevated, whence the profile falls very regularly in a slightly
convex line. The jaws appear equal when open, but when closed the
upper one is a little the longest. Teeth velutine, with four
well-marked canines in the upper jaw, two on each side of the
anterior extremity, the outer one of which is longer than the inner.
Opercle with two flat blunt points. Denticulations of the
preopercle, particularly those at the angle below the notch,
moderately well developed. Tubercle of the interopercle prominent.
There are scales on the cheeks and pieces of the gill-cover, but
none on the crown, snout, jaws, or suborbitals. The scales on the
body are moderately large. There are rows of small scales between
the rays of the vertical fins, but they are more developed between
the soft rays than between the spinous. The dorsal has the first
spine half the length of the second, which itself is a little
shorter than the third; fourth, fifth, and sixth equal and longest;
there is very little difference in the lengths of the remaining
rays, nor is there much between the spinous and soft portions of the
fin, which, taken as a whole, appears nearly even throughout. Anal
short, commencing in a line with the fourth soft ray of the dorsal,
and terminating at the same distance from the caudal as that fin:
second and third spines very stout. Pectorals narrow and pointed, a
little shorter than the head. Ventrals a little shorter than the
pectorals.
B. 7; D. 10/14, the last double; A. 3/8, the last double; C. 17, and
some short ones; P. 16; V. 1/5.
Length 6 inches.
COLOUR.—“Upper part pale lead-colour: pectorals yellow; ventrals and
anal orange: sides very pale yellow.”—D. In spirits, the colour
appears almost uniform greyish-white. The dorsal and anal fins have
an edging of black, which is not noticed by Mr. Darwin, and which is
characteristic of the species. The caudal is entirely dusky. There
are no traces of spots on any part of the body.
Habitat, Keeling Island, Indian Ocean.
All the known species of _Diacope_ are stated by Cuvier and
Valenciennes, as coming from the Indian seas. The _D. marginata_ was
first brought from thence by Commerson. It was afterwards received by
the authors above mentioned from Pondicherry. The expedition under
Captain Duperrey, met with it at the Island of Oualan. Mr. Darwin’s
specimen was obtained at the Keeling or Cocos Islands: I believe it to
be referrible to this species, as it possesses the characteristic black
edging on the dorsal and anal fins; but as the description in the
“Histoire des Poissons” is very brief, containing a mere notice of the
colours, I have thought it advisable to annex that of the present
individual.