REPORT ON THE MOLLUSC A. 37 



In the Lucinidtu of Fiacher, and in the sj^ecies of Cri/pfvdu)i of the Challenger (II), 

 the outer plate of the gill is not transformed into a dorsally directed " appendage," but 

 has altogether disappeared. For here again, as in Tdlina, the branchial plate directed 

 ventrally is homologous with the internal plate of the typical gill (E). In fact, its 

 recurrent or reflected lamina is internal. The embryological facts also show, according 

 to Lacaze-Duthiers,' that this plate is indisputal )ly the internal. 



From what has been said, it is not necessary to conclude, as Dall - has done, that 

 the gills cannot be employed for purposes of classification. They may be so used, but 

 in so doing we attend, as Eay Lankester ^ has shown, to their structure, and not, as 

 Fischer did, to their number. All the Pelecypoda have, in fact, on each side only a single 

 gill, each plate of which corresponds to half of the gill of Gastropod or Cephalopod. 

 This can be very clearly seen in primitive gills like those oi Mallet ia. But this single gill 

 may be greatly modified in the great majority of Pelecypoda, either by enlargement or 

 by reduction. Thus, as has been already explained, the gills come to have an appear- 

 ance quite difierent from those of other Molluscs. 



In a preceding publication ^ I was not in a position definitely to discuss Fischer's 

 classification, which was not then fully published. But since the publication of Fischer's 

 Manual, and my recent examination of a large number of Pelecypoda, I have become 

 convinced that his classification is unnatural. It tends, that is to say, to separate forms 

 so closely allied as Lucina and UnguUna, Tellina and Psammohia, Aspergillum and 

 Fistulana, &c., to place Malletia, Yoldia, and Nncula among Tetrabranchs, and 

 Solenomya among Dibranchs, though the gills in all cases are formed in essentially 

 similar fashion ; and, finally, to rank genera so peculiar as those of the " Anatinacea," 

 studied above, which are destitute of true gills {sensn sfricto), among the " Diliranchs," 

 in which the gills are well developed. 



The supposed parallel classification of Dibranchs and Tetrabranchs, which Fischer ^ 

 suggests, shows nothing more than that the same reduction of gill may occur in different 

 groups. 



Apart fi-om the two great works of Poli ^ and of Deshayes,' of which the former is 

 already very old, the general morphology of the Pelecypoda has not been the subject 

 of works extending over the entire group, or even over the greater part of it. Of late 

 years it has been rather to the physiology of these organisms that investigation has been 



1 Memoire sur le developpenient des brancliies des MoUusquea Acepliales Lamellibrauches, Ann. Sci. Nat. (Zuol), 

 ser. 4, t. V. p. 46. 



2 Report on the Mollusca, Bull. Mvs. Comp. ZovL, t. xii. p. 281. 



* Mollusca, Eucycl. Brit, 9th ed. t. xvi. p. 691. 



* Notice sur les Mollusques recueillis par M. le Capitaine Storms dans la region du Tanganyka, Bull. Mii.f. B">j. 

 Hist. Aat. Bil<i., t. iv., 1886, p. 120. 



5 Manuel de Conchyliologie, p. 1141. * Testacea utriusque Siciliae. 



' Histoire naturelle des Mollusques (Exploration de rAlgeric). 



