130 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



Genus.— APLOCHITON. Jen. 



Corpus elongatum, compression, subfusiforme, undique nudum alepidolum. Caput par- 

 vum. Rostrum breviusculum, subacutum. Os terminate, rictu modico. Maxilla 

 superior margine ex ossibus intermaxillaribus oinnino formato, maxillaribus, hcec 

 sub(Equantibus, retroductis. Denies minuti, acuti, in maxilla utrdque uniseriati, in 

 lingua et vomere longitudinaliter biseriati, in ossibus palatinis nulli. Apertura 

 branchialis amplissima, membrand triradiatd, subtus profundi emarginatd. Pinnce, 

 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 Cyprinidce 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 Salmonidce, 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 Siluridce, 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 Cyprinidce, and of the usual form ; the subopercle also is 

 very distinct ;t 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 cnr\oos simplex, et yj.rwv tunica. 



t Valenciennes says, in his preface to the fifteenth volume of the " Histoire des Poissons," that none of the 

 Siluridw have the subopercle ; and that the absence of this bone serves to distinguish them from Colitis. 



