246 MOLLUSCA. 



but are otherwise like those of the Aplysite ; the rest of their organisation is 

 always the same. 



AKKRA. Muller. 



The branchiae covered as in the preceding genera, but their tentacula are so 

 shortened, widened, and separated, that they seem to be totally wanting, or 

 rather to form a large fleshy and nearly rectangular shield, under which are 

 the eyes. The shell of such as have any is more or less convoluted, but 

 with little obliquity, and is without a projecting spire, emargination, or 

 canal ; the columella, projecting convexly, gives a crescent-like figure to the 

 aperture, the part opposite to the spire being always the broadest and most 

 rounded. 



Lamarck names those in which the shell is concealed in the thickness of the 

 mantle, Bi I.L.KA. It has but very few whorls, and the animal is much too 

 Targe to be drawn into it. 



Bullce aperta, Lam. The animal is whitish, and about an inch long; the 

 fleshy shield, formed by the vestiges of its tentacula, the lateral swellings 

 of its foot, and the mantle occupied by the shell, seem to divide its upper 

 surface into four lobes. Its thin, white, semi-diaphanous shell, is nearly all 

 aperture, and its gizzard is armed with three very thick rhomboidal pieces 

 of bone. It is found in almost every sea, where it lives on oozy bottoms. 

 The 



GASTROPTERON, Meckelii. 



Appears to be an Akera, the margin of whose foot is extended into broad 

 wings, used in natation, wlu'ch it effects on its back. It has no shell, nor 

 has the stomach any armature ; a slight fold of skin is the only vestige of a 

 branchial operculum that is perceptible. 



GASTROPLAX, Blainville. UMBRELLA, Lamarck. 



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 projecting and trenchant edges, along t!-e whole of the front and of 

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

 branchus, are under this slight margin. Under this same margin and for- 

 wards, are two tentacula, longitudinally cleft, as in Pleurobranchus, at whose 

 internal base are the eyes ; between them is a kind of proboscis. 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 surface 

 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 middle, with 

 trenchant edges, and marked with slightly concentric stria. 



