2 GENUS THRACIA. 
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 trachee, cylindrical, fleshy, approaching each other 
towards their origin; these two trachee can be extended far 
out of the shell ; the lower is larger and longer, and serves to 
supply the branchie 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 papille, 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 trachee 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’ is 
voluminous, and is terminated by a small, oval foot, compress- 
ed, fringed in form of a crest, and sub-anterior. ‘The branchie 
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 branchie is floating and free, and is 
continued as far as the entrance of the siphon. The lamelle — 
are fine and very contiguous, undulated, and a little oblique 
1 Tn the specimen we have before us, the abdominal mass is very volu- 
minous, and the ovary is seen beneath its covering. 
