356 



MOLLUSCA. 



tentacula, longitudinally cleft as in Pleurobranchus, and at their inner bases are the eyes : between 

 them is a kind of proboscis, perhaps an organ of generation. There is a large concave space in the 

 anterior margin of the foot, the edges of which can be drawn together like the mouth of a purse ; and 

 at its bottom is a tubercle pierced with an orifice, which is perhaps the mouth, and is surmounted by 

 a fringed membrane. The inferior surface of the foot is smooth, and serves the animal to crawl on, as 

 in other Gasteropodes. It carries with it a hard, flat, irregularly-rounded shell, thickest in the centre, 

 with sharp margins, and lightly marked with concentric striae. It was supposed at first that the shell 

 was attached to the foot, but more recent observations have proved that it is upon the cloak, and in its 

 usual place. 

 [Two species have been discovered: one in the Indian Ocean, the other in the Mediterranean.] 



THE FIFTH ORDER OF THE GASTEROPODES. 



THE HETEROPODA, Lam.* 



The Heteropoda are distinguished from all other Mollusca 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 several species, is a sucker in the form of a hollow cone, that 

 represents the disk of the other orders. Their branchiae, formed of plumose lobes, are situ- 

 ated on the hinder part of the back, and point forwards; and immediately behind them are 

 the heart and liver, of inconsiderable size, with a portion of the viscera and the interior organs 

 of generation. The body, of a transparent gelatinous substance, sheathed with a muscular 

 layer, is elongate, and generally terminated with a compressed tail ; the mouth has a muscular 

 mass and a tongue garnished with little hooks ; the gullet is very long ; the stomach thin ; 

 two prominent tubes, on the right side of the bundle of the viscera, serve as passages to the 

 excrements, and to the eggs or semen. They swim, in ordinary, in a reversed position ; and 

 they can innate the body with water in a manner which is not yet well understood. 



Forskal comprised them all under his genus Pterotrachea, which it is necessary to subdivide. 



The Carinaria, Lam., — 

 Has the nucleus (formed by the heart, the liver, and organs of generation,) covered with a thin, sym- 

 metrical, conoid shell, with the point curved 

 backwards, and often raised into a crest; under 

 its anterior margin, the plumes of the branchiae 

 float ; on the head are two tentacula, and the 

 eyes are behind their roots.f 



One species (Car. cymbium, Lam.) inhabits the 

 Mediterranean; another the Indian Ocean (Car. 

 fragilis, B. St. Vincent). The Argonauta vilrea of 

 authors may be a Carinaria, but its animal is un- 

 known. 



The Atlanta, Lesueur, — 

 From the observations of M. Rang, should be 

 animals of this order, whose shell, in place of 

 being expanded, has a narrow cavity, and a 

 spire rolled up on the same plane : its con- 

 tour is raised into a thin crest. They are very 



small shells of the Indian Sea ; and in one of them, Lamanon believed that he had found the original 



of the Ammonites. 



Fig. '73. 



-Carinaria : the shell of Its natural size, and a reduced figure of 

 the animal, with the position ot the shell 011 it. 



* M. ile ISIa'.iwille makes a family of this order, which he names 

 Nectoptida, and unites them in his Nncleobrumhiata with another 

 family named the Pteropoda, comprising, however, only Limacinaof 

 my Pteropodf s. He adds to it, upon 1 know not what conjecture, the 



Argonauta. [Sowerby has also contended for Argonauta beinir ar- 

 ranged near to Carinaria.] 



f Ste a description of the animal by M. Verony in the Zool. Jjurn. 

 vol. v p. 325.—ED. 



