REPORT ON THE MOLLUSCA. 35 



One remarkable exception to this riile is found in the forms of Solenomya. In 

 these Molluscs the gills apparently retain the simple structure characteristic of 

 MaUetia, but the lamellae are elongated, and one row extends towards the dorsal, and 

 the other towards the ventral surface. 



In Nucula (PI. IV. fig. 10, B) the beginning of the elongation of the lameUse may- 

 be already observed. A more advanced stage, showing the two rows of filaments 

 (elongated lamellae) simple and perfectly parallel, is not represented by any modern 

 forms. Solenomya would approximately represent this phase, if the two series of 

 lamellse, instead of remaining in the same plane (turned in opposite directions), were 

 both turned ventrally, that is to say, if the dorsal row were folded between the mantle 

 and the ventral series. But among the other extant Pelecypoda no form retains the 

 stage with simple filaments (PI. IV. fig. 10, C). 



In fact, in the gills which are least removed from those of Nucula, these filaments 

 become recurrent. Elongated to increase their surface of hsematosis, and not being 

 able to extend farther in a ventral direction, the filaments are turned upon themselves 

 in a dorsal direction, growing towards the point of origin of the axis, and exhibiting in 

 transverse section the form of an elongated V- Such is the structure of the gill in the 

 Arcidae (PL IV. fig. 10, D). The two series of filaments then form two branchial 

 plates. According to Dall,' Area eetomata possesses only one of these plates; but the 

 fact seems to me to demand corroboration. 



In the Arcidas these filaments are still entirely free and perfectly distinct from 

 one another. But in a more advanced stage of specialisation all the filaments of 

 one half plate or lamina {a or a', for example, in PL IV. fig. 10), though not fused 

 to one another, are nevertheless united by perpendicular bridges, while transverse 

 bridges connect the two branches of the same filament. This is the state of afi'airs 

 in Mytilus. 



In a more elaborated union and completer fusion, the filaments come to form four 

 laminae, of which the two internal and the two external are united to one another Ijy 

 longitudinal contact, leaving between them " interbranchial " tubular spaces, and by 

 transverse septa. In this way the gill consists of an external plate (with two laminae) 

 and of an internal plate (also with two lamina3). 



The surface of the plates may then become folded, and exhibit the most general 

 type of gill in the Pelecypoda (PL IV. fig. 10, E). This type may be further modified 

 by enlargement or by reduction. 



Thus in a large number of Pelecypoda, as, for example, in Cardium (PL IV. 



1 Eeport on the Mollusca, Bull Miis. Comp. ZooL, xii. p. 2-14. According to Dall, Dimrja (ihid., p. 232) and 

 Amusium (p. 210) have also only a single gill ou each side ; but I have shown that in the Challenger species of Amusiinn 

 there were two normal gills on each side. 



