18 THE VOYAGE OF H.M.S. CHALLENGER 



the mantle are joined. Tims the gills, being in juxtaposition to each other behind the 

 foot (by the reflected lamina of the inner or ventral plate), and to this fold (by their 

 posterior extremity, and the dorsal edge of their dorsal or external plate), effect a 

 complete separation between the two chambers. 



Dorsally to the gill there is on the mantle a glandular swelling, comparable in its 

 position to the hypobranchial gland of Gastropoda. 



19. Lyonsidla papyracea, Smith. Station 157; 1950 fathoms. 



As in the preceding species, the mantle has three apertures. The largest of these 

 is the branchial aperture, which here also is surmounted by a crown of tentacles, larger 

 and less numerous than in the above (PL II. fig. 8, q), and which is continued inwards 

 by an annular membranous valve similar to but more extended than that in Lyonsiella 

 jeffyeysi (k). This aperture is separated from the pedal aperture by a pallial com- 

 missure of considerable extent, as far as j. 



The mouth (a) is encircled by a fold (corresponding to the lips and palps), 

 specially developed anteriorly, where it stretches over the anterior adductor muscle. 



The foot (d) is cylindrical and obtuse at its distal extremity ; it has a simple 

 byssal groove on its posterior surface. 



The gill consists, as in Lyonsiella jejfreysi, of two plates, of which the dorsal or 

 outer (e') has only one lamina, while the ventral (e) has two. But here the gill is 

 thicker, the lamellae more dilated, and pressed close together, so as to give the organ 

 an almost fleshy appearance. 



The outer plate is joined to the mantle, all along its external margin, by means 

 of a membrane (A). This union is complete from the anterior adductor (/) to the 

 posterior addu<^'tor {m). In addition, the two gills are united to each other behind the 

 foot by the ventral margin of their ventral plate, and their posterior extremity is 

 joined, by the same membrane (h), to the division between the two siphonal apertures, 

 so that they form a great partition through which the foot passes, the reflected lamina 

 of the ventral plates forming a fold round it. 



We find, therefore, that the pallial cavity is divided into two chambers morpho- 

 logically similar to those which I have already had occasion to describe in Cryp)todon 

 moseleyi. But here the " anal " chamber is very much larger, for it extends to the 

 anterior adductor, that is, in front of the foot, of which the greater part is contained 

 within it. 



The pallial wall of the anal chamber exhibits on each side, as in Lyonsiella 

 jeffyeysi, a glandular thickening, which I have compared to the hypobranchial gland 

 of Gastropoda. 



These two species of Lyonsiella difler remarkably in regard to the arrangement 

 of the gills, and the existence of well-divided pallial " chambers." While in Lyonsiella 



