Notes on the Classification of the Corliculadce, etc. 423 



mantle lobes are lengthened, they become broad, they gape, 

 and they constitute the rear cavity of the syphons. Finally, 

 above the posterior adductor, the lobes are joined anew, they 

 become thickened, they follow on between the beaks and con- 

 tinue, still united, towards the anterior and superior side, where 

 they are introduced between the cardinal teeth of the shell. 

 The mantle has two large openings, an anterior one for the exit 

 of the foot, and a posterior one for that of the syphons. 



Owing to the proximity of the adductors to the ventral mar- 

 gin, the pedal aperture of the mantle is less long, than in 

 Galatea for instance, in which genus the adductors are nearer 

 the beaks. 



The interior of the mantle is provided with a row of small 

 conical tentacles, which are barely visible. 



§ III. — The muscular arrangement. The anterior adductor 

 is crescent-shaped, the convex side being towards the anterior 

 margin of the shell ; the posterior adductor is oval, the major 

 axis being about vertical, and it is surmounted by the retrac- 

 tory pedal muscle. There is a palleal muscle, which is quite 

 broad, of medium thickness, and is constituted of radiating 

 fibres. A bundle of strengthening fibres starts from the infe- 

 rior and anterior side of the anterior adductor proceeding 

 towards the dorsal side of the mantle. The same conformation 

 holds good at the junction of the palleal muscle with the pos- 

 terior adductor. The shell, on examination, shows no well- 

 defined palleal sinus, but the palleal line rises a little before it is 

 united to the impression of the posterior adductor. There being 

 no sinus in Corbicula, it was thought that in this genus there 

 existed no retractory syphonal muscle, and that consequently 

 there were no syphons. Such, however, is not the case, the 

 retractory syphonal muscle is present, though very rudimen- 

 tary, and is so intimately connected with the palleal muscle 

 that it is difficult to limit it. It may, however, be recognised 

 by the rounded outline of its anterior margin, and by the thick- 

 ening of the fibres. 



