336 Div. 2. MOLLUSC A. 



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

 viscera, through the envelopes. 



The Pnedmodermes {Pneumodermon, Cuv.) — 

 Carry their dissimilarity to the Clios a little farther. 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, (hsposed in the 

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

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

 fleshy tentaculuni. 

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



TiiE LiMACiN^, Cuv., — 

 Ought, from the description of Fahricius, to have a nigh relationship toPneumodermon; 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 (CV(o /if?('r/«a of Pliipps and of Ginel. ; Arffonaula arctica, Fahr., Faitn. Greenl. 387) is not 

 less abundant than the CUo borcalis, 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 tcntacula; a cloak slit on the sides, containing the branchiae at the 

 bottom of the fissures, and clothed with a shell slit in a con-esponding 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 fiu'uished with three acute denticulations. \Vhen alive, the animal jjrotrudes, through 

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



The best known species (Anomia Iridcniaia, Forskahl ; Carolina iiatans, AbiUlgaard ; 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 Ilyales iu the 

 simplicity of their wings, and the absence of tcntacula 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 germs into subg-enera thus :—C7<;o(;om, with the shell pyramidal; C/-rac/*, with the 

 slicll conical, elongated ; Ciiricrhi, 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 Tripier of Quoy and Gaimard, which Iflainville iias referred to the family Acerse.) 



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



The Pyugo, — 

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

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



(Several I'tcropodes have been discovered in the fossil state. M. Hang has found, in the ierrains 

 of Bordeaux, Hyales, Cleodorac, and Cuvicria;. — ?>caJnn. des i>ci. Nat. for Auijusl 182G. The ]'af/inula 

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



THE THIRD CLASS OF MOLLUSCA. 

 THE GASTEPvOPODES. 



T'ne Gasterojiods 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 frout> is more or 



• Sowerby unys, " Umbilicated on both sides."— Eo 



