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Show 130 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. GENUs.-APLOCHITON. Jen. Co1'pus elongatum, compressum, subfusifm·me, undique nudum alepidotum. Caput parvum. Rost1·um bre1Jiusculum, subacutum. Os terminale, rictu modico. Maxilla superior mm·gine ex ossibus inte1·maxillaribus omnino formato, maxillaribus, ltmc subcequantibus, ret1·oductis. Dentes minuti, acuti, in maxilla utraque 1miseriati, in lingud et vomere longitudinaliter biseriati, in ossibus palatinis nulli. Apertura brancltialis amplissima, membrana triradiata, subtus profunde emarginattl. Pinnce, dorsalis paululum pone ventrales, analis paululum pone dm·salem t·eclinatam, exorientes. Ventrales appendicibus axillaribus nullis. Pinna caudalis bifurca. I have already noticed a remarkable new form among the Cyprinidm 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 ft·ee from scales. There are, however, many other points of similarity between Mesites and the genus which I have here termed Aplocltiton.• 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 operde 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 Aplocltiton. Furthermore, the fins are on the whole very similar, with the exception of the dorsal not being so far back in Aplocltiton, 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, Aplocltiton might by some be referred to the Siluridte, but what was said of the genus Mesites may be repeated here, that it has none of the othet· 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 Cyprinidm, 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 ot· pectoral fins. At the same time it must be mentioned that Aplocltiton • Ab a?r:\oos simplex, ct x<r6JV tunica. t Valenciennes says, in his preface to the fifteenth volume of the "Ilistoire des Poissons" that none of the Siluridw have the suboperole; and th11t the absence of this bone serves to distinguish them f;om Cobitis. |