676 SIR c. ELIOT ON KUDiBRAMCHS [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 triangulai' plates of somewhat varying size. Other parts 

 of this digestive tract are mottled with reddish-brown spots which 

 bear papillee. 



The buccal mass was removed from the second specimen, which 

 was not further dissected in order to preserve its external 

 appearance. 



PLBUEOPHYLLIDIIDiE, 



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 Avhen once discovered they may be found in 

 considerable plenty and variety. 



Four genera have been described — Pleurophyllidia Meckel, 

 Linguella Blainville, Gamarga Bergh, Pleurophyllidiella Eliot ; 

 and to them I think should be added Pleuroleura ( = Dermato- 

 hranchus). Bergh and others make this last genus the type of a 

 separate family, which differs from the Pleui-ophyllidiidfe only in 

 having no gills or lamellae under the mantle. But as the organi- 

 sation of the two groups is the same and somewhat remarkably 

 difierent from that of other Nudibranchiata, it seems better to 

 recognise their afSnities by uniting them in one family, just as 

 has been done in the case of Tethys which has branchial tufts, and 

 Melihe which has none. 



in Pleiirophyllidui the anterior dorsal margin forms a continuous 

 fold behind the rhinophores, so that the space in front of the 

 rhinophores is sepai'ated 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 rhino- 

 phores 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 Pleurophyllidia, the real structure 

 being visible only on careful examination. It would seem that 

 when the animals are put into spirits the head is withdiawn 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 Pleurophyllidia has arisen in the course 

 of evolution. Linguella is probably the more primitive form, for 

 in Nudibranchs the i-hinopbores generally rise out of the dorsal 

 surface and it is the exception that they should be separated from 

 it. But if the rhinophoi'es come close together and the anterior 

 parts are often retracted, there must be a natural tendency to 

 form a fold as in Pleuroj^hyllidia. 



