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



We may therefore confirm Dall's opinion,' and conclude without hesitation that 

 Ver'ticordia tornata is a Poromya. 



Silenia.^ 



21. Silenia sarsi, Smitli. Station 157; 1950 fathoms; and Station 325; 2650 



fathoms ; two specimens. 



On lifting up one of the lobes of the mantle (PI. III. figs. 8, 9) of a Silenia, we see 

 an arrangement similar to the one observed in the Poromya just described. 



The great pallial ventral (pedal) aperture extends to {j), that is, to near the branchial 

 aperture. The posterior apertures are surrounded by a series of tentacles, fifteen in 

 number, of which one is dorsal (q). Tlie anal aperture is prolonged by a small project- 

 ing siphon (p) ; the branchial aperture has the same large internal tubular valve as the 

 Poromya Qc). 



The mouth (a) has two pairs of labial palps ; the anterior very large [h), the 

 posterior (c) small. 



The foot {(J) is rather long and linguiform, but has no byssal groove. 



The essential difference is in the aspect of the partition (e). It is disposed just as 

 in Poromya ; that is, it extends from the anterior adductor muscle (/) to the division 

 between the two posterior apertures. It is crossed by the foot, and, dorsal to it, there 

 is a chamber not entirely filled by the visceral mass (/). This partition has also 

 special muscles attaching it to the shell. 



But instead of the two groups of lamellae, separated by long slits, which we saw 

 on each side in Poromya granulata and Poromya tornata, we find here that the slits 

 piercing the partition are comparatively short, and that the lamellae have disappeared 

 to become the lips of the two apertures which they separate (see PI. III. fig. 10). 



Besides, these apertures do not form two groups on each side, as'in the preceding- 

 genus ; they form three : an anterior {g) , comprising five apertures in one specimen 

 (Station 325), and six apertures in the other (Station 157) ; a second group [g) towards 

 the middle of the partition, and behind the foot {g"), comprising five apertures ; and 

 finally, a third posterior group, consisting of three apertures. 



The two anterior groups are longitudinal, with the apertures transverse ; the last 

 group, on the other hand, is transverse (without being continuous, however, with the 

 corresponding group on the other side), and its apertures are longitudinal. The last 

 group {g") is situated on a rather large projection which the partition forms between 

 the foot and the division between the siphons. 



' Report on the Mollusca, Bull Mus. Comp. Zoiil, t. xii. p. 281. 



2 This title, due to Smith, ought to be ah creel, for it has beeu already used, in 1873, by Mulsant, for a coleop- 

 terous insect. 



