36 THE VOYAGE OF H.KS. CHALLENGER. 



fig. 10, F), the external lamina of the outer plate extends dorsally, so as to form 

 the begmning of a third lamella. This is what is called an "appendage." 



On the other hand, the whole of the outer plate may be reduced and directed 

 dorsally, as, for instance, in Tellina (PI. IV. fig. 10, G). This portion is then also 

 called an "appendage." But it is in nowise comparable, as Fischer believed, to 

 the appendage of Cardium, or of other Pelecypoda with two branchial plates of 

 which the outer is appendiculate. For in the latter cases the appendage corresponds 

 to the entire outer plate of the typical gill (E). 



It is by regarding this appendage of Cardium and of other Pelecypoda with two 

 branchial plates, as a structure homologous with the so-called "appendage" of 

 Tellina, that Fischer* has come to consider the ventrally directed branchial plate of 

 Tellina as homologous with the outer plate of the Pelecypoda which he designates as 

 " Tetrabranch." 



But this branchial plate of Tellina has its recurrent or reflected lamina tntevnal. 

 If the plate were external, its recurrent lamina would also be external. It therefore 

 represents surely the internal branchial plate of the other Pelecypoda mentioned 

 above. 



This arrangement of the gill seen in Tellina occurs in the greater number of the 

 members of Fischer's group Anatinacese (in which Hancock" has also previously 

 supposed that the ventrally directed plate corresponded to the outer plate of other 

 Pelecypoda), and in the family Clavagellidas which he includes in his group. But in 

 these groups the outer plate of the gill (Fischer's " appendage "), very much reduced 

 in Pandora, only possesses the internal lamina (PI. IV. fig. 10, I, a), the "direct" 

 lamina of Lacaze-Duthiers ; the external or recurrent lamina is absent.' 



I cannot for a moment suppose that this disposition of the gills (G and I) 

 of Tellinidaj, Anatinacea, Clavagellidaj (that is to say, the " Appendiculate 

 Dibranchs " of Fischer), could be derived from the arrangement found in Solenomya 

 (BB), where the two branchial plates are also directed in opposite directions, 

 the one ventrally and the other dorsally (the dorsal plate being for Fischer the 

 "appendage"), but where neither of the plates exhibits any recurrent or reflected 

 portion. 



I would regard the gill arrangement of Tellina, &c., as a modification of the tj'pical 

 disposition ("Tetrabranch") (E). For each of the groups cited, one may find among 

 the " Tetrabranchs " neighbouring forms which hardly diff'er, except in the arrangement 

 of the outer plate of the gill. 



1 Manuel de Conchyliologie, p. 1141. 



2 Hancock, On the animal of Cliamostrea albida, Aim. (mil Mar;. Nat. Hial., ser. 2, vol. xi. p. lO'J ; Hancock, On the 

 animal of Myochama anomioidcs, ///('</. p. 289; Lacaze-Dutbiers, Morphologie des Acepbales, Arch, de Zvol. Expe'i:, 

 ser. 2, t. i. p. 715, pi. xxxvii. fig. 4 (Axjieryillimi). 



