12 THE VOYAGE OF H.M.S. CHALLENGER. 



Limopsis. 



3. Limopsis minuta, Philippi. Station 75 ; 450 fathoms. 



4. Limopsis pelagica, Si'miih. Station 106 ; 1850 fathoms. 



5. Limopsis lata, Watson. Station 169; 700 fathoms. 



The animal of Limopsis has not yet been described in detail. For the most part 

 it resembles that oi Pectunculus. 



One point is remarkable about the gill. Here, as in all the Arcidse, there are 

 two branchial plates formed by the filaments corresponding to the lamellae of the 

 Nuculidse. But, as in these last {Nucula, Yoldia, Malletia), the branchial axis is fixed 

 by a supporting membrane to the mantle near its posterior extremity. 



In Limopsis, on the contrary, as in all the other Arcidse, the axis is free over all 

 the part of it posterior to the posterior adductor muscle (PI. II. fig. 1, h). This axis, 

 which is thus the support of the gill, becomes comparatively solid and resisting, 

 and in Lim,opsis much more than in Area and Pectunculus it is greatly developed in 

 size (PI. II. fig. 1). 



In this figure representing Limopsis pelagica. Smith, the axis and the whole gill are 

 greatly contracted. In PI. II. fig. 2, a sketch of Limopsis m,inuta, Philippi, shows these 

 organs very nearly as they are arranged in the living animal. The branchial support in 

 Limopsis is chiefly formed by muscular fibres. 



Area. 

 Several species from great depths have been examined, notably — 



6. Area pteroessa, ^vaith. Station 246 ; 2050 fathoms. 



Their structure is like that of the littoral species, especially as regards the form of 

 the gills. 



Pectinacea. 

 The littoral species have pigmented ocelli on the edge of the mantle. It would be 

 interesting to know whether these organs are present in the deep-sea species. 

 Two genera have been examined, Pecten and Amusium. 



Pecten. 



7. Pecten philip2ni, 'R.eclnz. Station 75 ; 450 fathoms 



8. Pecten suhhyalinus, Smith. 



9. Pecten vitreus, Chemnitz. 



l Station 310 ; 400 fathoms. 



In the three species there exist pigmented ocelli of proportionately the same size as 

 in the littoral species. The gills are similar to those of the latter. 



