58 EOCENE MOLLUSCA. 



consisting of a horny dentated plate, placed transversely across the upper part, 

 and the sharp outer edge of which forms, as it were, the upper jaw. The cavity of the 

 mouth is furnished with a thin cartilaginous tongue, the anterior extremity of which 

 is of a flattened spoon-like form, and which plays against the edge of the horny plate, 

 answering the purpose of an under jaw. The remainder of the tongue is rolled up 

 into a tube closed at the end, and thickly covered with teeth, distributed in transverse 

 rows of various forms. The number of these teeth is almost incredible, amounting, 

 in one of the English slugs {Limax maximus) to nearly 27,000, and ranging in 

 several of the snails from 10,000 to upwards of 20,000.* A dentition of a similar 

 character prevails among the Branchiated Gasteropods ; and Professor Loven has 

 proposed the employment, for the purposes of classification, of characters taken from 

 the form and arrangement of the teeth. 



The free air-breathing Molluscs are, in some few instances, viviparous,! but, for 

 the most part, they are oviparous. The eggs are either enveloped in a skin, or are 

 covered by a hard calcareous shell, which, among the larger Bulimi and Achatinae, is 

 sometimes of considerable size. The larvae are in all cases shaped like the parent. The 

 generative organs present various modifications ; in some genera the animals are 

 unisexual ; but more generally they are hermaphrodite. 



These Molluscs are, with few exceptions, provided with hard calcareous shells, 

 which are sometimes either internal or partly concealed beneath the mantle, but more 

 generally are external, and large enough to contain the whole, or nearly the whole, 

 of the animal. In some genera the foot of the animal is provided with a calcareous 

 or horny operculum ; in others the animal is without this appendage, and in the genus 

 Clausilia, the purpose of the operculum is answered by a peculiar apparatus termed the 

 clausiuin. The external shells present many modifications in the proportions and 

 conditions, as well of the spire and volutions, as of the aperture and columella. Certain 

 of these forms are accompanied by corresponding peculiarities of organisation, and the 

 genera w r hich have been established for their reception may be considered types in this 

 order ; such are the genera Helix, Bulimus, Pupa, Succinea, Limnsea, Physa, Planorbis, 

 Cyclostoma, Helicina, Auricula, &c, and the Palaeontologist will have little difficulty in 

 distinguishing them. Other genera, however, have been proposed from time to time 

 on characters taken from modifications of these typical forms ; but a more intimate 

 acquaintance with the anatomy of the animals has latterly induced great caution in the 

 admission of these genera ; since, in many cases, the Malacologist, after the most 

 careful investigation, has failed to detect any peculiarity of organisation corresponding 



* For a more detailed account of the oral apparatus, the reader is referred to Mr. W. Thompson's highly 

 interesting " Remarks on the Dentition of British Pulmonifera," in the 'Annals and Mag. of Nat. Hist.,' 

 2d series, vol. vii, p. 86. 



f This is the case with some species of Helix, and with several species of Bulimus, for which Ferussac, 

 on this ground, proposed the genus Partula. 



