PLATE XXIII. fig. 1.
_T. corpore subrhomboideo, compressissimo, altitudine fere dimidium
longitudinis, pinnâ caudali exclusâ, æquante: osse maxillari
angusto, retrorsum arcuato: pinnâ dorsali triangulari, suprâ
ventrales accuratè exorienti; anali lævi, longâ, altitudine retrò
cito decrescente; utrâque plicis membranaceis, radiis
longitudinaliter adhærentibus, instructâ: squamis in lineâ laterali
46, in lineâ inter pinnas ventrales et dorsalem transversâ 17._
B. 4; D. 1/10; A. 2/30; C. 19, &c.; P. 13; V. 8.
LONG. unc. 4. lin. 7.
FORM.—Of a subrhomboidal form, the nape and back being much elevated,
whence the profile falls very obliquely and in nearly a straight
line. Greatest depth nearly half the entire length, excluding
caudal. Body very much compressed, the thickness being nearly three
and a half times in the depth. Head approaching to a laterally
flattened cone, with the length and height nearly equal. Snout very
short; mouth but little cleft; when open, the lower jaw projecting
considerably. Maxillary narrow, and of nearly equal breadth
throughout, curving backwards. Teeth with their cutting edges
dentated, the middle point much the most developed, with one or two
smaller ones on each side: two rows of such teeth on the
intermaxillary, and one in the lower jaw, this last row with
scarcely more than eight or ten teeth in it. No teeth on the
maxillary, vomer, palatines, or tongue. Eyes round, rather large,
their diameter three and a half times in the length of the head,
distant not so much as one diameter from the end of the snout.
Nostrils with two orifices, the posterior one a narrow curved slit,
the anterior one a round hole. The suborbital forms a somewhat
triangular naked disk beneath the eyes, with radiating veins.
Posterior margin of the opercle very little curved: subopercle
narrow, and small, forming but a small portion of the gill-flap.
About seventeen scales in the depth, and forty-six in the lateral
line, which last bends downwards rather below the middle, and is
continued quite to the caudal. A scale taken from the middle of the
side below the lateral line is somewhat rounded anteriorly, the
basal margin being straight; the surface marked with very fine
numerous concentric striæ, and with two coloured deeper striæ on the
free portion diverging from the centre in a V-like form: some scales
have three or four of these coloured striæ, drawn more or less
regularly.
The dorsal commences in about the middle of the entire length,
excluding the caudal and narrow part of the tail; of a triangular
form, its greatest height equalling the depth to the lateral line.
Pectorals narrow, shorter than the head, attached below the bottom
of the gill-opening, and reaching rather beyond the insertion of the
ventrals, which last are in a vertical line with the commencement of
the dorsal and shorter than the pectorals. Anal long, commencing a
very little beyond the tips of the reclined ventrals; the anterior
portion rather more than half the height of the dorsal, but the
posterior much lower, sloping rapidly off; two spines, the first
very minute, the second about one-third the length of the first soft
ray; the last soft ray double. The anal terminates nearly in a line
with the adipose, which is small. Caudal forked for half its length:
the lobes equal. Many of the rays in the vertical fins, more
especially the dorsal and anal, are accompanied through nearly half
their length from the bottom by membranous folds of skin. There are
also some small scales along the base of the anal, but none apparent
on the dorsal. In the axillæ of the ventrals is an elongated scale,
not half their length.
COLOUR.—“Back bluish silvery, with a silver band on the side: a bluish
black spot behind the gills. Fins pale orange; tail with a black
central band.”—D.—There is now not much trace of the silver band, or
the black band on the tail. The humeral spot is, however, still very
distinct.
Habitat, the Rio Parana, South America.
This species was taken by Mr. Darwin in October in the Rio Parana, as
high up as Rozario. I cannot ascertain that it is described, though
there is much resemblance between it and the species figured in
Seba.[35] It differs from the _T. chalceus_ of Spix, in its much smaller
scales, not to mention other points of dissimilarity.
2. TETRAGONOPTERUS RUTILUS. _Jen._
PLATE XXIII. fig. 2.
_T. corpore ovali, compressissimo, altitudine tertiam partem
longitudinis, hâc ad basin furcæ caudalis mensâ, æquante: osse
maxillari angusto, retrorsum arcuato: pinnâ dorsali subtriangulari,
paululum pone ventrales exorienti; anali lævi, longâ, altitudine
retrò cito decrescente; utrâque plicis membranaceis, radiis
longitudinaliter adhærentibus, instructâ: squamis in lineâ laterali
40, in lineâ inter pinnas ventrales et dorsalem transversâ 14._
D. 1/9; A. 2/27; C. 19, &c.; P. 14; V. 8.
LONG. unc. 4. lin. 3.
FORM.—More oval than rhomboidal: the back and nape not so much
elevated as in the last species; the profile falling less obliquely.
Depth one-third of the entire length, measured to the base of the
caudal fork: head one-fifth of the same. Not above fourteen scales
in the depth, and forty in the lateral line, which occupies the
eighth row from the top. The scales on the whole similar, but with
the basal margin not so straight and regular, and somewhat
projecting in the middle in the form of a blunt salient angle. The
dorsal commences a trifle nearer the caudal, and at the middle of
the entire length, the caudal alone excluded. The pectorals reach
just to the insertion of the ventrals, which last are a trifle in
advance of the dorsal. The second anal spine is longer, and nearly
half the length of the soft rays which follow.
COLOUR.—“Back iridescent greenish brown: a silver band on the side.
Fins dirty orange: tail with a central black band; above and below
the band bright red and orange.”—D.—The colours appear very similar
to those of the last species. The humeral spot, however, is less
obvious, while, on the other hand, the silver band on the side can
still be distinguished.
Habitat, Rio Parana, South America.
Taken with the last species, to which it is very closely allied. Mr.
Darwin observes in his notes, that both are among the commonest of the
river fry in the Rio Parana.
Both this and the _T. Abramis_ are distinguished by having narrow
longitudinal folds of skin attached to the basal half of the rays of the
dorsal and anal fins, a character which does not shew itself in any of
the three species next to be described.