344 MOLLUSCA. 



of a little beak. The transparency of the body allows us to distinguish the heart, the brain, and the 

 viscera, through the envelopes. 



The Pneumodermes {Pneumodermon, Cuv.) — 

 Carry their dissimilarity to the Clios a little further. The body is oval, without cloak or shell ; the 

 branchiae attached to the skin, and formed of little leaflets set in two or three lines, disposed in the 

 figure of the letter H opposite to the head ; the fins small ; the mouth (garnished with two small lips, 

 and two bundles of numerous tentacula, terminated each by a sucker) has underneath a small lobe, or 

 fleshy tentaculum. 

 The only species {P. Peronii, Cuv.) was taken in the ocean by Peron. It is not less than an inch in length. 



The Limacin^, Cuv., — 

 Ought, from the description of Fabricius, to have a nigh relationship to Pneumodermon; but their body 

 is terminated with a spiral tail, and is lodged in a very thin shell, of one whorl and a half, umbilicated 

 on one side,* and flat on the other. The shell serves the purpose of a boat ; and when the creature 

 wishes to swim on the surface, it uses its fins as oars. 



The species known (Clio hellcina of Phipps and of Gmel. ; Argonanta arctica, Fabr., Faun. Greenl. 387) is not 

 less abundant than the Clio borealis, in the Arctic seas ; and is hkewise a principal aliment of the Whale. 



The Hyales {Hyalea, Lam. ; Cavolina, Abildg.) — 

 Have two very large wings ; no tentacula ; a cloak slit on the sides, containing the branchiae at the 

 bottom of the fissures, and clothed with a shell slit in a corresponding manner, the ventral aspect of 

 which is very tumid ; the dorsal aspect is flat, longer than the other, and the transverse line which unites 

 them behind is furnished with three acute denticulations. When alive, the animal protrudes, through 

 the chinks of the shell, certain narrow filaments, or productions of the cloak, of variable lengths. 



The best known species (Anomia tridentata, Forskahl ; Carolina natans, Abildgaard ; Hyalea cornea, Lam.) has 

 a small yellowish semi-transparent shell, and is found in the Mediterranean and the Indian Ocean. 



The Cleodores {Cleodora, Peron), 



For these. Brown first instituted the genus Clio. They appear to be analogous to the Hyales in the 

 simplicity of their wings, and the absence of tentacula between them. It is also probable that their 

 gills are concealed in the cloak ; but their conical or pyramidal shell is not slit along the margins. 



M. Rang distributes the genus into subgenera thus -.—Cleodora, with the shell pyramidal ; Creseis, with the 

 shell conical, elongated ; Ctivieria, with the shell cylindrical ; Psyche, the shell globular; Euribia, the shell hemi- 

 spherical. (And it is probable that we should arrange near the Creseis, and even perhaps in the same subgenus, 

 the Tripter of Quoy and Gaimard, which Blainville has referred to the family Acerae.) 



It has been believed that we may place near to the Hyales, — 



The Pyrgo, — 

 A very small fossil shell discovered by M. Defrance. It is globular, very thin, and divided by a very 

 narrow transverse fissure, excepting in front, where it becomes also a little enlarged. 



(Several Pteropodes have been discovered in the fossil state. M. Rang has found, in the terrains 

 of Bordeaux, Hyales, Cleodorae, and Cuvieriae. — See Ann. des Sci. Nat. for August 1826. The Vaginula 

 of Daudin is a Creseis, according to Rang ; and it has, in fact, all the characters of the same.) 



THE THIRD CLASS OF MOLLUSCA. 



THE GASTEROPODES. 



The Gasteropods constitute a very numerous class, of which the Slug and the Snail give 

 a good general idea. They creep generally upon a fleshy disk, situated under the belly, 

 but which sometimes assumes the form of a furrow, or of a vertical lamina. The back 

 is covered with a cloak of greater or less extent, and of various figure, which secretes 

 a shell in the greater number of the genera. Their head, placed in front, is more or 



* Sowerby says, " Umbilicated on both sides."— Ed. 



