130 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 
Grenus.—APLOCHITON. Jen. 
Corpus elongatum, compressum, subfusiforme, undique nudum alepidotum. Caput par- 
vum. Rostrum breviusculum, subacutum. Os terminale, rictu modico. Mazilla 
superior margine ex ossibus intermaxillaribus omnino formato, maxillaribus, hec 
subequantibus, retroductis. Dentes minuti, acuti, in maxilla utrdque uniseriati, in 
lingua et vomere longitudinaliter biseriati, in ossibus palatinis nulli. Apertura 
branchialis amplissima, membrana triradiatd, subtus profund2 emarginatd. Pinne, 
dorsalis paululum pone ventrales, analis paululum pone dorsalem reclinatam, exori- 
entes. Ventrales appendicibus axillaribus nullis. Pinna caudalis bifurca. 
I have already noticed a remarkable new form among the Cyprintde 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 Salmonide, 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 Siluride, 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 de- 
veloped as in any of the Cyprinide, 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 
* Ab amXoos simplex, et yirwy tunica. 
+ Valenciennes says, in his preface to the fifteenth volume of the “ Histoire des Poissons,” that none of the 
Stluride have the subopercle ; and that the absence of this bone serves to distinguish them from Cobitis. 
ay 
