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 (Limar mazximus) 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 Lovén 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 Achatine, is 
sometimes of considerable size. The larvee 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 
clausium. 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 which have been established for their reception may be considered types in this 
order ; such are the genera Helix, Bulimus, Pupa, Succinea, Limnzea, Physa, Planorbis, 
Cyclostoma, Helicina, Auricula, &c., and the Palzontologist 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 
* Fora 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. 
+ This is the case with some species of Helix, and with several species of Bulimus, for which Férussac, 
on this ground, proposed the genus Partula. 
