286 



MANTJAL OP THE MOLLUSCA. 



aquatic tribes. Land-snails are universally distributed; but 

 the necessity for moist air, and the vegetable nature of their 

 food, favour their multiplication in warm and humid regions : 

 they are especially abundant in islands, whilst in 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. 

 Presh-water snails [Limnceidce] 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 obvious characters afi'orded by the animal and shell. 

 The operculated land-snails have seven-ranked teeth, like 

 Paludina and Littorina. The in-operculated air-breathers 

 have, without known exception, rows of very numerous, similar 

 teeth, with broad bases, resembling tessellated pavement. Their 

 crowns are recurved, 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 simila.r in shape ; curves 

 indicate gradual changes, and angles accompany sudden altera- 

 tions of form. 





/^ilv!^! 



v/iOlvluNI^Mv/lvjIwh 



I ^ i e I 



Fig. 122. Lingual teeth of Achatina.* 



m 



ii 



■ vliUi 



The absolute number of teeth is only a specific character, and 

 is usually greatest in the larger species ; but the Helicelke have 

 fewer teeth in proportion than the Helices, and Yelletia has 



* Fragmeut of tlie lingual membrane of Achatina fulica, with central and lateral 

 teeth more enlarged, from a specimen communicated by J. W. Laidlay, Esq. 



