Acanthurus humeralis, _Cuv. et Val._ Hist. des Poiss. tom. x. p. 170.
FORM.—General form oblong-oval. Greatest depth just behind the
insertions of the pectorals; contained exactly twice in the length
of the oval of the body (measuring this last from the end of the
snout to the base of the caudal spine), and three times in the
entire length (measuring this last to the extremities of the lobes
of the caudal fin.) Profile convex before the eyes, whence it
descends nearly vertically to the mouth. Height of the head a little
exceeding its own length. Eyes very high in the cheeks, and in front
of each a grooved line passing horizontally forwards towards the
nostrils; which last consist of two small round orifices, the
anterior one larger than the other, and partially covered by a
membranous flap. There are seventeen teeth in the upper jaw, and
sixteen in the lower: those above have the cutting edges crenated,
and likewise the lateral edges for nearly half-way down; this most
observable in the middle ones, in which the crenations amount to
eight or ten in number: those below similar, but with the crenations
not quite so numerous, and in some of the teeth at the sides of the
jaw almost confined to the cutting edges. Scales minute; those taken
from the middle of the body appear of an oblong form, their apical
portions dotted, and ciliated with from twelve to eighteen very
minute denticles, their surface marked with extremely fine delicate
striæ, not distinguishable without a strong lens.
The lateral line follows the curvature of the back; at about
one-fifth of the depth. The spine on the sides of the tail is
strong, and sharp-pointed, and very slightly bent. No reclined
spine before the dorsal. Both the fin just mentioned and the anal
have their soft portions terminating posteriorly in rather an
acute angle: also both have a scaly membrane at the base, and rows
of minute scales between the soft rays extending for about
one-third or more of their length. The first ray of the anal is
very minute, and so much concealed in the skin as to be easily
overlooked. The filaments of the caudal are sharp-pointed, and
extend as far again as the middle rays: the upper one rather
longer than the other. The pectorals are contained three times in
the length of the oval of the body. Ventrals attached a little
further back, sharp-pointed, and terminating in the same vertical
line with the pectorals, both being laid back.
D. 9/23; A. 3/23; C. 16, &c.; P. 16; V. 1/5.
Length, to the end of the caudal lobes, 7 inches.
COLOUR.—The colours appear to have been exactly as described in the
“Histoire des Poissons.” Mr. Darwin’s notes taken from the recent
fish state, “splendid verditer blue and green;” but do not enter
into the details of the markings.
Habitat, Tahiti.
Obtained at Tahiti, where it had been previously found by MM. Lesson and
Garnot. Mr. Darwin’s specimen accords with the characters given by
Cuvier and Valenciennes, except in having two soft rays less in the
dorsal, and one less in the anal. Their description, however, is not
very detailed.