REPORT ON THE MOLLUSCA. 17 



Anatinacea. 



Lyonsiclla. 



The only anatomical knowledge we possess about this genus is due to the 

 researches of Sars,^ who was the first, and hitherto almost the only, investigator of the 

 structure of deep-sea Molluscs. 



There are two species belonging to this genus in the Challenger collection, 

 Lyonsiella Jeffrey si, Smith, and Lyonsiella pai^yracea, Smith. 



18. Lyo7isiella jeffreysi, Smith. Station 106 ; 1850 fathoms. 



The mantle has three openings, an anterior, of moderate size (pedal), a large 

 posterior, ventral (branchial), and a small posterior (anal). 



The anal aperture forms a short siphon which is invaginated in the only existing 

 specimen (PI. II. fig. 7, jj). The branchial aperture bears a crown of papilla-like tentacles, 

 somewhat similar to those figured by Sars {loc. cit., pi. iii. fig. 40) from Lyonsiella 

 ahyssicola. This aperture is continued inwards by a circular membranous fold 

 projecting into the pallial cavity, and probably with a valvular function. 



The foot contracted in the specimen is cylindrical and rather narrow ; near its 

 base is a projection, ventral to which is the aperture of the byssus. 



The mouth {a) has neither palps nor lips in the strict sense of these words. That 

 is to say, there is no distinction possible between lips and palps. The mouth is 

 encircled by a fold more expanded in front (the part corresponding to the anterior 

 palps) than behind. A gill arises fi'om each side of the posterior portion of this fold. 



This gill is delicate, and seems to be formed of a single plate. But, as in the 

 other Anatinacea (I do not include Solenomya in this group), there are two branchial 

 plates turned in contrary directions, like the two halves of an open book. The 

 dorsal or outer plate (the so-called " appendage ") is somewhat reduced, and has only 

 one (the direct) lamina. The ventral plate has two laminae, but the reflected lamina 

 is already reduced, and its most distal portion is no longer in folds. 



The greater portion of the gill is free. The anterior portion of the dorsal plate 

 is united to the mantle, for a short distance, by its dorsal edge. But the right and 

 left gills are not united to each other behind the foot, so that the pallial cavity is not 

 divided into two chambers definitely separated by the gills. 



But this division into two chambers can be eff'ected in the following manner : 

 a fold arising between the two posterior pallial apertures (PI. II. fig. 7, h) stretches into 

 the pallial cavity to the distal extremity of the gills, with which, however, it does not 

 unite, and is continued from each side, over the mantle, to the point where the gill and 



' On some Remarkable Forms of Animal Life. i. p. 27 (1872). 

 (zooL. CHALL. Exr. — PART Lxxiv. — 1888.) Eece 3 



