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



glands, &c. And as the name Cryptodon was created specially for Lucina Jlexuosa, 

 our species cannot remain in this genus, and a new name must be invented for them. 



After what I have said above, there can be no such doubts as Fischer has 

 expressed^ regarding the systematic position of the genus Axinus ( = Cryptodon, 

 properly so called). Since it has two branchial plates on each side, this genus ought 

 to be placed in the family Ungulinidse ; and it has, besides (according to Sars), only 

 one siphonal aperture. 



The characteristic of a single siphonal aperture, which Fischer emphasises for the 

 Ungulinidse, will not hold good (if this family is adhered to), because the Diplodonta 

 have two posterior apertures. I must add that Axinus Jiexuosus should be studied in 

 detail, for Forbes and Hanley" say that it has two apertures, and no tube ; while 

 according to the brothers Adams,^ Axinus has a single tubular, siphonal aperture, and 

 Woodward* attributes to it a lonsf anal tube. 



The new genus established for our " Cryptodon " should be placed, then, in the 

 " family" Lucinidse, though it has only one siphonal aperture like Ungulina. 



The comparative study of these two " families," Lucinidee and Ungulinidse, shows 

 that the Pelecypoda cannot be classified according to the number of then- branchial 

 plates. A classification based on this characteristic would be quite artificial, since it 

 would separate all those animals in which the rest of the structure, and even the 

 structure of the gills, is very similar, and which, taken together, would form such a 

 natural group, Liicina, " C7'yp>todon," Diplodonta, Axinus, Ungulina. 



We shall see later that this classification, based on the number of branchial plates, 

 has no real basis. 



Tellinacea. 



Semele { = Syndesmya, Eecluz). 



16. Semele pro/undorum, Smith. Station 244 ; 2900 fathoms. 



I have figured (PI. II. fig. 6) a specimen of this species to show the relative 

 proportions of the labial palps and the gills. In the littoral species of this genus the 

 palps are never so large as the gills. Here, on the other hand, we see the gill 

 (e and e') greatly reduced and much smaller than the palps (b), which have preserved 

 their normal size. 



17. Semele longicallus, Scacchi. Station 75 ; 450 fathoms. 



In this species the arrangement is exactly the same as in Semele p)rofundorum. 



' Manuel de Conchyliologie, p. 1098. 



- History of the British Mollusca and their Shells, vol. ii. p. 469. 



^ The Genera of Recent Mollusca, t. ii. p. 56. 



* A Manual of the Mollusca (1856), p. 293. 



