_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_.[37] 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;[38] 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.
[Illustration:
_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.