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Show G76 SIR C. ELIOT ON NUDIBRANCHS [June 19, The outer teeth approach the hamate shape, but all have the double row of denticles. In the specimens already dissected by Alder and Hancock were found in the stomach (presumably the second stomach) twelve yellowish triangular plates of somewhat varying size. Other parts of this digestive tract are mottled with reddish-brown spots which bear papillae. The buccal mass was removed from the second specimen, which was not further dissected in order to preserve its external appearance. P l e u r o p h y l l i d i i d j e . The Oriental collections belonging to Alder and Hancock contain twelve specimens of this family referable to six species. This is an unusually large number. It is probable that the animals burrow in sand or mud and escape the notice of ordinary collectors, though when once discovered they may be found in considerable plenty and variety. Four genera have been described-Pleurophylliclia Meckel, Linguella Blainville, Ccimarga Bergh, PleurophyllidieUa Eliot; and to them I think should be added Pleuroleura (= Dermato-brcmchus). Bergh and others make this last genus the type of a separate family, which differs from the Pleurophyllidiidae only in having no gills or lamellae under the mantle. But as the organisation of the two groups is the same and somewhat remarkably different from that of other Nudibranchiata, it seems better to recognise their affinities by uniting them in one family, just as has been done in the case of Tethys which has branchial tufts, and Melibe which has none. In PleurophylUdia the anterior dorsal margin forms a continuous fold behind the rhinophores, so that the space in front of the rhinophores is separated from the rest of the dorsal surface. In Linguella, on the other hand, the anterior fold is not continuous, and the dorsal surface passes between the rhinophores without interruption, and then spreads out into the area in front of the rhinophores. Though this character is clear and decisive in well-preserved specimens, it is often obscured by distortion, and most of the Linguellce in the present collection have superficially the appearance of Pleurophylliclia, the real structure being visible only on careful examination. It would seem that when the animals are put into spirits the head is withdrawn and contracted, with the result that a fold is formed behind the rhinophores. This artificial fold may illustrate the manner in which the natural fold of PleurophylUdia has arisen in the course of evolution. Linguella is probably the more primitive form, for in Nudibranchs the rhinophores generally rise out of the dorsal 'surface and it is the exception that they should be separated from it. But if the rhinophores come close together and the anterior parts are often retracted, there must be a natural tendency to form a fold as in PleurophylUdia. |