160 MANUAL OF THE MOLLUSCA. 



their antiquity might be inferred from the distribution of peculiar genera in 

 remote islands, associated with the living representatives of the ancient 

 fauna of Eui'ope. Fresh-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 in- 

 stance specifically, identical with living types of the new world {Megaspira, 

 Proserpiva, Glatidina, and Helix lahyr'mthica). In the coal-strata of Nova 

 Scotia, Sir Chas. Lyell has discovered a single specimen of a reversed and 

 striated shell, apparently a Claiisilia. 



The lingual dentition of the pulmonifera confirms, in a remarkable manner, 

 those views, respecting the affinities of the order, and its zoological value, 

 which have been deduced from the more obvious characters afforded by the 

 animal and shell. The operculated land-suails have seven-ranked teeth, like 

 Pahidina and Litorina. The in-operculated air-breathers have, ^^•ithout 

 known exception, rows of very numerous, similar teeth, with broad bases, 

 resem])ling tessellated pavement. Their croA^ms 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 nmnber 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 ac- 

 company sudden alterations of form. 



/ c I 

 Tig [)0 Lingual teeth of Achahna.* 



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

 greatest in the larger species ; but the Helicella have fewer teeth in propor- 

 tion than the Helices, and Velletia has fewer than Ancylus. The anomalous 

 genus Amphilola (p. 139) has an unusually broad tongue, armed with teeth 

 similar to those of the snail. 



Fig, 91. Lingual teeth of Amphibola.i 



About one-third the lingual membrane is spread over the tongue ; the rest 

 has its margins rolled together, and is lodged in a sac or dental canal, which 



* Fragment of the lingual membrane of Achatina fulica, with central and lateral 

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



t Part of the tongue of AmpMbola avellana, from a preparation by J. W. Wilton, 

 Esq., of Gloucester. 



