286 MANUAL OF THE MOLLUSCA. 
aquatic tribes. Land-snails are universally distributed; but 
the necessity for moist air, and the vegetable nature of their 
food, fayour their multiplication in warm and humid regions: 
they are especially abundant in islands, whilst m hot and 
desert countries they appear only in the season of rain or dews. 
Their geological history is less complete than that of the purely 
marine orders; but their antiquity might be inferred from the 
distribution of peculiar genera in remote islands, associated with 
the living representatives of the ancient fauna of Europe. 
Fresh-water snails (Limneide) occur in the English Weald, 
but fossil land-snails have not been found in strata older than 
the tertiary in Europe, and then under forms generically, and 
even in one instance specifically, identical with living types of 
the New World (Megaspira, Proserpina, Glandina, and Helix 
labyrinthica). In the coal-strata of Nova Scotia Sir Charles 
Lyell has discovered a single specimen of a reversed and striated 
shell, apparently a Clausilia. 
The lingual dentition of the pulmonifera confirms, in a re- 
markable manner, those views respecting the affinities of the 
order, and its zoological value, which have been deduced from 
the more obyious characters afforded by the animal and shell. 
The operculated land-snails have seven-ranked teeth, lke 
Paludina and Littorina. The im-operculated air-breathers 
have, without known exception, rows of very numerous, similar 
teeth, with broad bases, resembling tessellated pavement. Their 
crowns are recuryed, and either aculeate or dentated. The 
lingual ribbon is very broad, often nearly as wide as it is long; 
and the number of teeth in a row (though usually a third less) 
is sometimes as great, or even greater, than the number of 
rows. The rows of teeth are straight or curved or angulated ; 
when the rows are straight the teeth are similar in shape ; curves 
indicate gradual changes, and angles accompany sudden altera- 
tions of form. 
Sicha 
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Fig. 122. Lingual teeth of Achatina.* 
The absolute number of teeth is only a specific character, and 
is usually greatest in the larger species; but the Helicedle have 
fewer teeth in proportion than the Helices, and Velletia has 
* Fragment of the lingual membrane of Achatina fulica, with central and lateral 
teeth more enlarged, from a specimen communicated by J. W. Laidlay, Esq. 
