14 THE VOYAGE OF H.M.S. CHALLENGER 



differs from the cylindrical, claviform, posteriorly-directed foot of the Luciuidai. Here 

 the principal part of the foot is directed forwards, swollen, and terminating in a point ; 

 behind this part is a byssal aperture (PI. II. fig. 4, e). 



The mouth (a) is Ixirdered anteriorly and posteriorly by two rather narrow lips 

 (h), which are continued on each side in small, bifid, labial palps {PL II. figs. 3, 4, c). 



There is only one branchial plate (g) on each side, as in Lucina. This organ is 

 triangular in form, and has no dorsal "appendage." It is fixed by its anterior edge, 

 and by the beginning of its dorsal edge, to the base of the dihedral angle formed by 

 the union of the mantle with the visceral sac. 



Behind the foot the recurrent or internal laminae of the two gills are united to each 

 other by a membrane (PI. II. fig. 4, h), which joins their dorsal edges. The common 

 posterior extremity of the two gills is united at the separation of the anal aperture 

 from the great branchio-pedal aperture (PI. II. fig. 3, r), so that the pallial cavity is 

 divided into two distinct spaces, — a large, ventral, infra-branchial space (the branchial 

 chamber, properly so called) ; and a small, dorsal, posterior, supra-branchial space (anal 

 chamber), into which the anus opens, and which communicates with the exterior liy the 

 anal aperture ; while the inferior or branchial chamber communicates with the exterior 

 by the great pallial, ventral, branchio-pedal aperture. 



This division of the pallial cavity is present in a certain number of other 

 Pelecypoda; but in several of these, such as Mija, Pholas, Pliodon,^ &c., the 

 separation is absolute, while in others ( Unio, Anodonta,^ those with one siphon like 

 " Cri/ptodon" moseleyi, Myochama,^ &c.) there persists between the posterior portion 

 of the visceral mass and the gills a narrow opening by which the two chambers 

 communicate with each other. 



In the same way, in the " Cnjptodon " alluded to above, one can pass a probe 

 introduced at the anal aperture through a similar opening into the ventral chamber 

 (PL II. fig. 4, k). 



It may be remarked, that in many forms with two siphons this division into two 

 chambers does not exist, and that, as the posterior extremity of the gills does not 

 become united to the partition between the two siphons, these both open into the great 

 undivided pallial cavity (Tellinidse, Donacidee, Psammobiidse, &c.). 



Deshayes* makes out that the gills of Lucina, which are similar to those of 

 " Cryptodon " moseleyi, have two laminae, corresponding to the two branchial plates 

 of the typical Pelecypoda. The gill of Lucina and Cryptodon moseleyi does indeed 



' Paul Pelseneer, Notice sur les Mollusques recueillis par M. le Capitaine Storms dans la region du Tanganyka, 

 Bull. Mits. Rny. Hist. Nat. Bch/iijue, t. iv. p. 117. 



- Woodward, A Manual of the MoUusca (1856), fig. 171, b; Ray Lankester, Mollusca, Encycl. Brit., 9th ed., vol. xvl 

 p. 686, fig. 124, 5, g. 



^ Hancock, On the Animal of Myochama Anoraioides, Ann. Mag. Nat. H/s/., ser. 2, vol. xi., pi. xi. fig. l,f. 



■* Remarques sur I'organisation des Lucines, Comptes rendits, t. xx. (1845) p. 1704 ; Traite elementaire de 

 conchyliologie, t. i. part 2, p. 707. 



