MOLLUSCA, 109 



eyes at the tips of the two longest. This contains the fol- 

 lowing British species, viz. bidens, perversa, biplicata, plica - 

 tula, and labiata. The other genus, called VERTIGO, was 

 formed by Miiller. The animal possesses only two tenta- 

 eula, with the eyes on their tips. The T. vertigo is the 

 type of the genus. 



28. HELIX. Linnaeus, in constructing this genus, at- 

 tended only to the character of the mouth being contract- 

 ed or lunated, without regarding the habits of the animals, 

 or even the other forms which the shells exhibited. Hence 

 he has united globose, discoid, and turreted, terrestrial, flu- 

 viatile, and aquatic shells ; animals with two and with four 

 tentacula, with and without an operculum, oviparous and 

 viviparous. 



The marine species of Linnaeus are few in number. The 

 genus JANTHINA of Lamark, has been formed from the H. 

 janthina of Linn, a species of which has lately occurred at 

 several places of the Irish coast. The opening is triangu- 

 lar, and there is an angular sinus at the right edge. The 

 shell, which Linnaeus terms H. haliotoidea, is completely 

 concealed in the animal. There are many marine shells 

 inserted in the genus Helix by British writers, which either 

 belong to the restricted genus Turbo, or to the Vermicu- 

 laria. 



The further reduction of the Linnaean helices depends 

 on the separation of the terrestrial from the fluviatile shells, 

 and subdividing these according to the characters furnUh- 

 ed by the different groups. 



Among the terrestrial shells, the restricted genus HELIX 

 is by far the most extensive. It contains those shells which 

 are subglobose, with a convex spire ; the opening entire, 

 wider than long, and diminished in its upper part by the 



