Otolithus guatucupa, _Cuv. et Val._ Hist. des Poiss. tom. v. p. 56. pl.
104.
FORM.—Elongated, with the back only very slightly elevated beneath the
first dorsal: in advance of that fin the dorsal line is nearly
straight, and continuous with the profile. Greatest depth contained
exactly four times and a half in the entire length. Head long,
contained three and a half times in the same. Lower jaw projecting
considerably beyond the upper, and ascending to meet it. Two
strongly developed curved canines at the extremity of the upper jaw;
the rest of the teeth in this jaw consist of a single row of fine
card, nearly equal throughout: in the lower jaw there are no
canines, but one similar row of card, rather stronger than those
above, and not equal, the smallest being in front, and those at the
sides becoming gradually larger as they extend backwards. No vestige
of scales on the lower jaw, lips or maxillary; but the suborbital is
covered with bright silvery scales. Eye full, and moderately sized;
its diameter one-fifth the length of the head; its distance from the
end of the upper jaw equalling the diameter. Margin of the
preopercle with a few indistinct striæ and obsolete denticulations.
Opercle with two flat points not much developed. Lateral line very
distinct, commencing at rather less than one-third of the depth, but
curving gradually downwards to one-half; continued to nearly the
extremity of the caudal; each scale marked with an elevated line,
from which there proceed one or two small ramifications on each
side.
First dorsal triangular, with the first spine very short, the fourth
longest, the fifth and succeeding ones gradually decreasing, the
last or tenth being shorter than the first. Second dorsal almost
contiguous, its spine or first ray about equalling the first ray of
the first dorsal: this fin is more than half as long again as the
first, and the rays are nearly even. The anal commences further back
than a point opposite the middle of the second dorsal; there are in
reality two spines in this fin, but the first is so extremely minute
as to be almost microscopic, and not seen, unless very carefully
sought for; the second or principal spine is weak, and rather more
than one-third the length of the soft rays. Caudal apparently
square, but the rays being worn at the tips, its exact form cannot
be determined. The second dorsal, as well as the anal and caudal,
are partially covered with small scales, which, however, are not
very obvious. Pectorals narrow and rather small, being scarcely more
than half the length of the head. Ventrals placed a little further
back, and rather shorter than the pectorals.
B. 7; D. 10—1/20; A. 1/8; C. 17; P. 16; V. 1/5.
Length 9 inc. 9 lines.
COLOUR.—“Silvery white, above iridescent with violet purple and
blue.”—D. Mr. Darwin has not noticed the dark transverse lines,
which descend from the back obliquely forwards, as represented in
the ‘Histoire des Poissons,’ and of which there are evident traces,
though apparently much effaced by the action of the spirit.
Habitat, Maldonado Bay, Rio Plata.
This species, which Cuvier and Valenciennes consider as the _Guatucupa_
of Margrave, was obtained by Mr. Darwin at Maldonado. M. D’Orbigny had
previously taken it at Monte Video. The only respects in which Mr.
Darwin’s specimen differs from D’Orbigny’s, is in its having two more
rays in the soft dorsal, and a slightly longer anal spine, judging from
the figure in the ‘Histoire des Poissons;’ but I cannot imagine that
they are distinct on these grounds only, so exactly do they agree in all
their other characters.