78 



with the horizontal septum, which continues forward the 

 fusion of the mantle edge between the two siphons, and 

 consequently they form the floor of the ventral supra- 

 branchial chamber which is a continuation forward of the 

 cavity of the dorsal siphon. This ventral suprabranchial 

 chamber is continuous with a series of cavities between 

 the two lamellae of the right and left inner branchiaB. 

 The reflected lamellae of the outer branchiae, on the other 

 hand, are filamentar in structure through all their width, 

 and extend dorsally beyond the ctenidial base to be 

 inserted into the body-wall in the angle formed by the 

 latter and the mantle lobes. This dorsal extension of the 

 reflected lamellae of the outer branchiae is common to a 

 number of Eulamellibranchs, of which Cardium forms 

 the type, and is the "appendice" of Pelseneer, which is 

 regarded by him as the beginning of a third lamella of the 

 outer branchia.* 



The branchiae of Cardium are of the " fluted or com- 

 pound type," that is, each is thrown into a number of deep 

 furrows and ridges, the direction of which is perpendicular 

 to the base of the ctenidium. Two such ridges, with a 

 furrow between, are represented in fig. 26 — where the 

 branchia is cut in a plane at right angles to the filaments. 

 Each ridge consists of a group of from 20 to 30 filaments. 

 One or two filaments may occupy the summit of the ridge, 

 the bottom of the furrow is constituted by two filaments 

 which have opened out and fused by their edges forming a 

 membranous plate. In each branchia these ridges and 

 furrows are symmetrically opposed to each other, ridge 

 against ridge, and furrow against furrow. The supra- 

 branchial cavity extends into the interior of each ridge. 



Morphologically the branchia consists only of this 



* Bull. Sci. de la France et de la Belgique. Ser. III., t. XX., pp. 27 — 52 

 1889, 



