MOLLUSCA CONCHIFERA. 
Crass III. GASTEROPODA. 
ANIMAL anticé capitatum; capite plus minusve prominulo, oculis ten- 
taculisque szepissimé instructo ; ore nudo, exsertili, seepits partibus 
duris armato; pallio amplo, vario, plerumque libero, testam pro- 
creante. Branchie varie, rard symmetrice, vel per aérem vel per 
aquam respirantes. Circulatio duplex corde uniloculari, auriculis 
duabus, valdé remotis, interdum diviso. 
Testa calcarea, rarO cornea, sepissimé spira regulari convoluta, 
plerumque operculo plus minusve clausa ; interdum simplex animal 
obumbrans, nonnunquam lamina testacea sustentata ; interdum sim- 
plex, parva, pro branchiarum tutamine intra pallio celata. 
In dividing the great series of Mollusca according to the variations of 
the organ of locomotion, a very considerable majority of them are found to 
belong to the present class. The Gasteropoda or ventral-moving mol- 
lusks, the Cephala of Deshayes, the Paracephalophora of De Blainville, in- 
clude all those which acquire motion by the contraction and dilatation of a 
flat, fleshy, expanded, ventral disc. The greater part of them havea spiral 
body, inclosed in a spiral shell, and are separated by Lamarck under the 
title of Trachelipoda or neck-moving mollusks ; we shall not, however, 
pursue this arrangement, because the transition from the straight body as 
in Limaz to the spiral body as in Helix, is too gradual to admit of their 
being separated. In following out the method of Cuvier, it is curious to 
observe the changes in the relative distribution of the genera, as compared 
VOL. I. B 
