Pimelodus gracilis, _Val._ in D’Orb. Voy. dans l’Amer. Mérid. Atl.
Ichth. Pl. 2. fig. 5.
———— _Cuv. et Val._ Hist. des Poiss. tom. xv. p. 134.
FORM.—Of a slender elongated form, the body compressed behind the
dorsal. Greatest depth contained about seven and a half times in the
entire length: thickness at the commencement of the dorsal a little
less than the depth. Head, measured to the gill-opening, rather more
than one-sixth of the entire length: its breadth two-thirds of its
own length. Helmet smooth, and not very conspicuous, though with its
whole surface finely wrinkled: its breadth behind the eyes rather
more than one-third of its length, measuring this last from the end
of the snout to the further extremity of the interparietal process.
The solution of continuity extends back nearly to the base of the
process just mentioned, which last is narrow and lanceolate, three
times as long as broad at its base, but not reaching to the buckler,
or triangular plate in front of the dorsal, by one-third of its own
length. The buckler itself is not very large, but sufficiently
obvious.
Profile sloping gradually downwards in nearly a straight line from the
beginning of the dorsal to the end of the snout: this last depressed
and rounded horizontally in the form of a semicircle. Mouth wide,
but very little cleft, the commissure not reaching half-way to the
eye. Upper jaw projecting a very little beyond the lower. In each a
band of very fine velutine teeth; but none on the vomer or
palatines. Tongue smooth, and fastened down all round. Six barbules;
the maxillary pair very long, reaching to the commencement of the
anal fin; of the submandibular pairs, the exterior reach one-third
beyond the insertion of the pectorals; the interior are only half
the length of the exterior. Eyes round, of moderate size, their
diameter four and a half times in the length of the head, situate in
about the middle of the length: distance from one to the other one
diameter and a quarter. Lateral line nearly straight throughout its
course, dividing the body longitudinally into two nearly equal
portions.
Pectorals not quite equalling the length of the head, and a little
less than one-sixth of the entire length: the spine very little
shorter than the soft rays, very strong, with sharp teeth on its
inner edge, but the outer edge only granulated, or with a few slight
serratures towards the extremity. The humeral bone seen above the
pectoral projects backwards in the form of a spinous lamina, but
does not appear through the skin; it equals half the length of the
pectoral itself. The dorsal commences at one-fourth of the entire
length, and is of a somewhat rectangular form, the soft rays not
decreasing much backwards: its length equals four-fifths of the
depth of the body, and two-thirds of its own height. The spine is
not so strong as that of the pectoral, and with only a few small
serratures on the outer edge near the tip. The space between the
dorsal and the adipose a little exceeds the length of the former.
The adipose itself is twice the length of that fin; very low at
first, but gradually rising, until, before its termination, it
becomes equal to between one-half and one-third of the depth. The
vent is in the middle of the entire length, caudal excluded. Anal
short, and just beneath the middle of the adipose, there being about
one-fourth of this last fin in advance of it as well as behind it:
the first four rays simple, but apparently all articulated, the
first two or three very minute and not easily observed. Caudal
forked for two-thirds of its length: the upper lobe a little longer
than the lower, and contained five and a half times in the entire
length. Ventrals immediately beneath the last ray of the dorsal; a
little shorter than the pectorals, and not reaching to the anal by
half their own length.
D. 1/6; A. 14 or 15; C. 17, &c.; P. 1/9; V. 6.
Length 5 inches 2 lines.
COLOUR.—(_In spirits._) Brownish, inclining to silvery in some places:
a dusky fascia formed of dots along the lateral line. Dorsal rather
dusky at the base, and with the upper portion also dusky between the
rays: a dusky spot on the anterior part of the adipose.
Habitat, Rio de Janeiro.
This species was taken by Mr. Darwin in a running brook at Rio de
Janiero. It approaches on the whole so nearly the _P. gracilis_ of
D’Orbigny, that I can hardly suppose it to be distinct. Yet there are
some slight differences observable in this specimen. It has more anal
rays; the adipose appears shorter; and the upper lobe of the caudal is
not so prolonged, though possibly it may be worn down. Also D’Orbigny’s
figure appears to want the dusky stains on the dorsal and adipose fins.
If it be not that species it must be new, as there is none other
described by Cuvier and Valenciennes with which it will assimilate
better.