350 MOLLUSCA. 



Gastroplax, Blainv. Umbrella, Lam. 



Until the anatomy of this singular genus be more closely investi- 

 gated, we are compelled to place it among the Tectibranchiata, and 

 even near Pleurobranchus. The animal is large and circular, the 

 foot projects considerably beyond the mantle, and its upper surface 

 is studded with tubercles. The viscera are in a round, superior, 

 and central part. The mantle is only visible by its slightly pro- 

 jecting and trenchant edges, along the whole of the front and of the 

 right side. The lamellated pyramidal branchiae, like those of Pleu- 

 robranchus, are under this slightmargin, and behind them is a tubu- 

 lar anus. Under this same margin and forwards, are two tentacula, 

 longitudinally cleft, as in Pleurobranchus, at whose internal base are 

 the eyes; 'between them is a kind of proboscis, which may possibly 

 be the organ of generation. There is a large concave space in the 

 anterior margin of the foot, the edges of which are susceptible of 

 being drawn up like the mouth of a purse, and at the bottom of 

 which is a tubercle, pierced by an orifice, which perhaps is the 

 mouth, and surmounted by a fringed membrane. The inferior sur- 

 face of the foot is smooth, and serves the animal to crawl on, as in 

 the other Gasteropoda. 



The shell is stony, flat, irregularly rounded, thickest in the mid- 

 dle, with trenchant edges, and marked with slightly concentric striag. 

 It was at first thought to be attached to the foot, but more recent 

 observation has proved that it is on the mantle and in the usual 

 place(l). 



(1) In the specimen from the British Museum described by M. de Blainville, 

 Bullet. Phil., 1819, p. 178; by the name of Gastroplax, the shell is, in fact, 

 attached to the under part of the foot, and by what means it is difficult to deter- 

 mine; the mantle, however, is so thin, that it seems as if it must have been pro- 

 tected by the shell. M. Reynaud has just brought to France a specimen which 

 had lost its shell, but where, it appears, traces of the membranes which attached 

 it to the mantle can be perceived, notwithstanding which, no remains of muscles 

 are visible. A similar shell is also found in the Mediterranean; its animal, how- 

 ever, has not yet been observed. 



