Part 37

The above description is that of two small specimens, similar to each other, obtained by Mr. Darwin in a running brook at Socego, in the province of Rio de Janeiro. They are probably not full grown; but so evidently distinct from either of the last two species, the characters of which are in some measure combined in them, that I have not hesitated to give them a separate place. The silver band is more or less obvious in all the species of this genus brought home by Mr. Darwin, but it is much brighter in this than in any of the others.

5. TETRAGONOPTERUS INTERRUPTUS. Jen. PLATE XXIII. fig. 4.

T. corpore ovali, valde compresso: altitudine tertiam partem longitudinis totius æquante: ore parvo; osse maxillari brevissimo, dilatato, margine posteriore recto: dentibus minutis, multicuspidatis: dorsali subtriangulari, pone ventrales exorienti; anali (in uno sexu?) scabrâ, altitudine retrò subito decrescente; his pinnis plicis membranaceis nullis: squamis in lineâ longitudinali 35, in lineâ inter pinnas ventrales et dorsalem transversâ 10: lineâ laterali interruptâ, haud finem pinnæ pectoralis attingente.

D. 1/10; A. 2/18; C. 18, &c.; P. 11; V. 7. LONG. unc. 2. lin. 8.

FORM.—Oval, and not very dissimilar in general form to the T. rutilus, but rather more elevated above the shoulder. Depth exactly one-third of the entire length: head one-fourth of the same, caudal excluded. Profile not exactly straight, but very slightly hollowed out at the crown, then as slightly protuberant above the eyes, whence it falls more rapidly in front, giving the snout a short and blunt appearance. Mouth much smaller than in any of the preceding species, owing to the extreme shortness of the maxillary, which is broad, with the anterior margin curving outwards, but the posterior one straight. Teeth very small; the points on the cutting edges numerous (five or six on each tooth) and nearly equal; apparently only one row on the intermaxillary, and the same in the lower jaw; none on the maxillary. Eyes and nostrils as in the other species, but the anterior orifice of the latter larger. Subopercle much larger, forming a greater portion of the gill-flap. Scales relatively larger; only ten in the depth, and thirty-five in the length. They have no deep striæ on the basal portion, and consequently no fan; but they are very regularly marked with the usual finer striæ concentrically arranged, except on the free portion where they become indistinct.

The lateral line occupies the sixth row of scales from the top, but is very soon interrupted, coming to an end before it has reached the length of the pectoral, and not carried over more than eight or nine scales in the length. Dorsal subtriangular, commencing exactly at the middle of the length, caudal excluded. Anal shorter than in the other species, and not reaching so near the caudal; two spines, but the first extremely minute. Caudal forked for half its length, the lobes equal. Ventrals more forward than in the other species, decidedly in advance of the dorsal, and attached at one-third of the entire length; their axillary scale very small. Pectorals narrow, reaching beyond them. No long folds of skin accompanying the rays of the vertical fins.

COLOUR.—Not noticed in the recent state, in which, judging from its appearance in spirit, it was probably silvery, or perhaps golden, with somewhat of an olivaceous hue becoming deeper on the back. There are evident remains of a bright longitudinal lateral band: also of a black spot at the base of the caudal. The dorsal and the anterior portion of the anal incline to dusky: the pectorals and ventrals are slightly dusky at their extremities; there is also a large irregular dusky stain on the back and shoulders anterior to the dorsal fin.

A second specimen differs from the above in being a trifle smaller, and not quite so deep in the body. The anal is also decidedly scabrous, as in the T. scabripinnis, and has one ray less in it. The colours are similar, excepting that the fins are rather less dusky, and the large dusky stain on the back and shoulders is wanting.

Habitat, Maldonado.

This species is one of several that were taken by Mr. Darwin at Maldonado, in the lake that had been suddenly drained, before alluded to. It is immediately distinguished from all the others in this genus above described, by its small mouth and abbreviated lateral line. The circumstance of the anal fin being scabrous in only one of the specimens leads to the suspicion that this may be a sexual character, perhaps common to this and several species; and, judging from its somewhat less depth, I conceive the specimen so distinguished in this instance to be a male.

HYDROCYON HEPSETUS. Cuv.

Hydrocyon hepsetus, Cuv. Reg. An. (ed. 2) tom. ii. p. 312. ———— falcatus, Freycinet, (Voyage) Zoologie, p. 221, pl. 48. fig. 2.

FORM.—Back rising slightly from the nape, whence the profile in front falls obliquely in nearly a straight line to the mouth. Depth contained about three and a half times in the length, caudal excluded. Both head and body much compressed, the greatest thickness being only two-fifths of the depth. The length of the head equals the depth of the body. Snout appears rather pointed when the jaws are shut; when open, the lower jaw is a little the longest. Gape considerable. Maxillary long, commencing before the eyes, and reaching to a vertical from the posterior part of the orbit; inclining downwards, lapping obliquely in part over the lower jaw, gradually widening towards the posterior extremity, which is rather obliquely rounded. Intermaxillary with two sharp canines in front, then on each side four or five very small hooked teeth, then another large canine, though not so large as those in front; behind this commences the maxillary, which is armed all along its margin with a regular row of small equal hooked teeth, resembling sharp serratures; a similar row on each palatine, but none on the vomer or tongue; this last pointed, and free at the tip. Lower jaw with two strong canines in front, larger than those in the upper, and fitting into two holes above, when the mouth is shut; on each side of these are three only half their size, but increasing backwards, placed at rather wide intervals; then follow a row of close, minute, sharp teeth, similar to those on the edge of the maxillary. Eyes rather large, their diameter not quite one-fourth the length of the head, distant one diameter and a quarter from the end of the snout. Suborbital large, consisting of three pieces. Preopercle rectangular. A row of pores, not very distinct, along the under part of the lower jaw, thence continued along the limb of the preopercle. Opercle and subopercle taken together with the posterior margin forming a slight but regular curve, with scarcely any salient angle.

Head naked; scales on the body of moderate size, arranged in somewhat oblique rows, especially below the lateral line; one from the middle of the side below the lateral line of an irregularly rounded form, the posterior margin rather sinuous, the disk with numerous fine concentric striæ, but no deeper-cut striæ on the basal portion. About sixteen scales in a vertical row, and fifty-seven or fifty-eight in the lateral line: this last bending downwards in a curve which falls below the middle of the depth. Scales on the lateral line not larger than the others.

The dorsal answers to the space between the ventrals and anal; its height equals the depth to the lateral line. Anal long, commencing exactly under the last ray of the dorsal; the first part of this fin as high as the dorsal, but the rays, beyond the fifth, gradually decreasing; three spines, the first two very minute; the last soft ray double. Caudal in this specimen injured. Adipose and last ray of the anal in the same vertical line. Pectorals two-thirds the length of the head, attached very low down beneath the terminating portion of the gill-flap, narrow and slightly falcate, reaching to the ventrals, which last are one-fourth shorter. A long narrow scale in the axilla of each ventral one-third the length of the fin itself.

B. 4; D. 11; A. 3/26; C. 22, &c.; P. 12; V. 8. Length 4 inches 3 lines.

COLOUR.—“Bluish silvery.”—D.—Some appearance of a dusky spot at the base of the caudal prolonged in a line along the middle rays, but scarcely any trace of a humeral one. The dorsal and anal incline a little to dusky.

Taken at Maldonado, in a fresh-water lake, in June. I have scarcely any doubt of its being the H. falcatus of the Zoology of Freycinet’s voyage, the figure of which it exactly resembles, excepting that the humeral dark spot, if it ever existed, and which is not mentioned in Mr. Darwin’s notes, is now almost entirely effaced. It is probable, however, that there are two or three species nearly allied, for which reason I have been the more particular in my description. The H. Hepsetus of D’Orbigny appears to differ from the H. falcatus of Freycinet (with which last Cuvier associates his name of Hepsetus,) in having the lateral line curving upwards rather than downwards, and the caudal fascia as well as the humeral spot more marked. The Salmo falcatus of Bloch is probably distinct from both.

GENUS.—APLOCHITON. Jen.

Corpus elongatum, compressum, subfusiforme, undique nudum alepidotum. Caput parvum. Rostrum breviusculum, subacutum. Os terminale, rictu modico. Maxilla superior margine ex ossibus intermaxillaribus omnino formato, maxillaribus, hæc subæquantibus, retroductis. Dentes minuti, acuti, in maxillâ utrâque uniseriati, in linguâ et vomere longitudinaliter biseriati, in ossibus palatinis nulli. Apertura branchialis amplissima, membranâ triradiatâ, subtus profundè emarginatâ. Pinnæ, dorsalis paululum pone ventrales, analis paululum pone dorsalem reclinatam, exorientes. Ventrales appendicibus axillaribus nullis. Pinna caudalis bifurca.

I have already noticed a remarkable new form among the Cyprinidæ brought home by Mr. Darwin, and differing from all the known genera in that family by the entire absence of scales. The one now to be described is not less remarkable among the Salmonidæ, and, what is particularly interesting, would seem to occupy an exactly analogous place in this family, departing from it in the same important character of having the skin perfectly naked and free from scales. There are, however, many other points of similarity between Mesites and the genus which I have here termed Aplochiton. In both there is the same form of mouth, the margin of the upper jaw being entirely formed by the intermaxillary, behind which is the maxillary of nearly equal development. The teeth in the jaws are similar, both in regard to form and arrangement; there is also the same double longitudinal row on the tongue, and along the vomer. The pieces of the opercle are similar, and the gill-opening equally large in both genera, though the branchial membrane has twice the number of rays in Mesites that it has in Aplochiton. Furthermore, the fins are on the whole very similar, with the exception of the dorsal not being so far back in Aplochiton, and there being also an adipose in this genus. It is also deserving of notice that both these new forms, so resembling each other in many of their characters, come from the same quarter of the globe, being found either in the most southern parts of S. America, or in the neighbouring islands.

From the circumstance of the naked skin, Aplochiton might by some be referred to the Siluridæ, but what was said of the genus Mesites may be repeated here, that it has none of the other external characters of that family. The maxillary, instead of being reduced to a mere vestige, or lengthened into a barbule, is as much developed as in any of the Cyprinidæ, and of the usual form; the subopercle also is very distinct; while there is no strong spine at the commencement of either the dorsal or pectoral fins. At the same time it must be mentioned that Aplochiton agrees with the Siluridæ in having no cœcal appendages, though the air-bladder is similar to that of the ordinary Salmonidæ. There are some peculiarities connected with the anal and sexual orifices which I shall notice presently, along with other points in the internal structure, in the species to be described first.

Fish. Pl. 24.

Waterhouse Hawkins. del. ]

1. Aplochiton Zebra. Nat. Size. 1a. Magnified show of anal and generative orifices. 2. Aplochiton tæniatus. Nat. Size.

Mr. Darwin has brought home two species of this new genus, agreeing precisely in form, but very distinct in respect to size and colouring.

1. APLOCHITON ZEBRA. Jen.

PLATE XXIV. Fig. 1.