tj GENUSTHRACIA. 



forms a slit for the passage of the foot ; the two others are 

 seen at the posterior part, and give passage to two distinct 

 tubes or tracheae, cylindrical, fleshy, approaching each other 

 towards their origin ; these two tracheae can be extended far 

 out of the shell ; the lower is larger and longer, and serves to 

 supply the branchiae with water: the upper, which is nearer 

 to the hinge, is shorter and straighter ; it serves for the excre- 

 mentitious evacuations ; it is furnished, at its upper and inter- 

 nal extremity, with two small tubercles, formed like teats, and 

 armed with small papillae, whose use is probably to prevent 

 the entrance of small foreign bodies. This trachea has no 

 communication with the lower; it is prolonged, moreover, into 

 the interior of the cavity, where it is continued by a delicate, 

 transparent valve, which extends even to the opening of the 

 anus. These tracheae can be drawn into the portion of the 

 mantle which surrounds them, the covering of which is loose, 

 and gives this part the appearance of a vulva : it is surrounded 

 by a tendinous, solid, and elastic ring ; a similar apparatus 

 exists for the passage of the foot. The abdominal mass 1 is 

 voluminous, and is terminated by a small, oval foot, compress- 

 ed, fringed in form of a crest, and sub-anterior. The branchiae 

 are voluminous, unequal upon the same side, and united in the 

 whole of their length, at the upper surface ; at the lower and 

 internal surface corresponding to this part, they are divided 

 into two, by a very distinct furrow ; they are thick, long and 

 wide, of a slightly oval form at the posterior part, and trun- 

 cated obliquely at the anterior part ; the lower being longer ; 

 the upper is adherent at its middle part in the two anterior 

 thirds near the union of this part with the lower ; this connex- 

 ion is indicated at the upper part by a slight ridge. The 

 posterior third of the two branchiae is floating and free, and is 

 continued as far as the entrance of the siphon. The lamellae 

 are fine and very contiguous, undulated, and a little oblique 



1 In the specimen we have before us, the abdominal mass is very volu- 

 minous, and the ovary is seen beneath its covering. 



