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Show 400 SIR C. ELIOT ON NUDIBRANCHS [Mar. 1, space behind. They are 24 in number: those in front are fairly large, but the size diminishes backwards, and those in the spirals are extremely small. The head and anterior portion of the foot are much retracted, but the latter- was apparently round and grooved in life. The mouth leads into an unusually large and spacious cavity, but the buccal mass is very small, the radula minute, and the oesophagus extremely narrow. The labial armature is small but strong, with rough projecting teeth ; the elements are small, yellowish, bent rods, mostly bifid. No rhachidian thickenings are visible; the innermost teeth have three denticles on the inner and about four on the outer side. The remainder have mostly five on the outer side only. The number of denticles increases towards the outside of each row, but the outermost teeth are smooth. The teeth are very small, crowded, and extremely numerous. This specimen corresponds with previous descriptions of C. atro-marginata, except that there are no thickenings on the rhachis of the radula and that the gills are much more numerous than in the specimens previously described. There seems to be some doubt on this point (see Bergh, Mus. Godeffr. 1. a), but the arrangement in the present specimen is perfectly clear. Casella cincta from Mauritius has 22 gills (Bergh, S. R. xvi. 2, p. 839), but presents differences in the coloration and buccal parts. Genus CERATOSOMA, Adams & Reeve. This genus is rendered unique among Nudibranchs by its extraordinary shape, but in its essential characters it is closely allied to Chromodoris. Indeed, if one looks at one of the high, stout Chromodorids (e. g. the figure of C. semperi in S. R. Heft xi. pi. lv. fig.' 2), it will be seen that one has only to somewhat prolong the tail and to thicken and develop the mantle-edge in order to obtain the characteristic shape of Ceratosoma. The large strong radula is like that of Chromodoris, but the denticles are minute and inconspicuous. The branchiae are much divided. Bergh recognises nine species, but I have only seen the descriptions of those examined by himself (cornigerum, gracillimum, trilobatum, oma-tum, polyomma). In Zanzibar I have inspected more than forty specimens which are apparently referable to one species, and are connected together by numerous gradations in colour and shape, though the extreme forms look remarkably different. As the five species mentioned above are distinguished by their external characters only, and offer no certain differences in the dentition or other organs, I am inclined to think they are merely varieties and that there is only one real species. It is to be noted, however, that though many of my specimens resembled the figure of Ceratosoma gracillimum (S. R. pi. xxv. fig. 8) the border was never red as there depicted, but always violet, and in no case did ocelli occur as in C. polyomma. |