258 MOLLUSCA. 



GASTROPTERON, Meek. 



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

 wings, used in natation, which 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, Blainv. UMBRELLA, Lam. 



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 the whole of the front and of 

 the right side. The lamellated pyramidal branchise, 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 striae. 



ORDER V. 



HETEROPODA, Lam. 



The Heteropoda are distinguished by their foot, which, instead of 

 forming a horizontal disk, is compressed into a vertical muscular 

 lamina, which they use as a fin, and on the edge of which, in seve- 

 ral species, is a dilatation forming a hollow cone, that represents the 

 disk of the other orders. Their branchiae, composed of plumiform 

 lobes, are situated on the hind part of the back, directed forwards, 

 and immediately in their rear are the heart and a small liver, with 

 part of the viscera. Their body, a gelatinous and transparent sub- 

 stance lined with a muscular layer, is elongated and usually termi- 

 nated by a compressed tail. There is a muscular mass belonging 

 to the mouth, and a tongue furnished with little hooks. They 

 usually swim on their back with the foot upwards. They have the 



