IIELICLNA. 801 



Operculum non-spiral, annular, seini-oval or sub-trian- 

 gular, with concentric elements, thick and testaceous, or 

 thin and horny. 



Shell with the aperture semilunar. 



In the character of their lingual dentition, as well as in 

 their organisation generally, these animals do not appear to 

 differ essentially from those of the Cyclophoridw ; the 

 tentacles, however, are more slender and produced, and the 

 caudal extremity of the foot is more elongate; the oper- 

 culum, moreover, is formed on an entirely different plau, 

 and the aperture of the shell, instead of being circular, is 

 semilunar in its outline. In their habits they are very 

 similar to the animals of the Cyclophorida;, but are con- 

 siderably more locomotive and lively ; in common with some 

 other mollusks, as the Neritida and Ellohiidxe, they pos- 

 sess the faculty of removing the internal septa and columella 

 of the shell, though by means of what agency is still un- 

 certain. 



Genus HELICINA, Lamarck. 



Operculum non-spiral, somewhat semi-oval, membranous 

 or testaceous. 



Shell heliciform, turbinate, globose or depressed, base 

 callous around the columella ; columella somewhat flattened, 

 rather straight ; aperture triangularly semi-oval, entire ; 

 peristome simple, straight or thickened, often widely ex- 

 panded. 



Syn. Ampullina, Blainv., not Lam. 



Ex. H. citrina, Grateloup, pi. 87, fig. 4. Operculum, 

 H. citrina, fig. 4, a, 4, b. Shell, H. neritella, Lamarck, 

 fig. 4, c. 



In Helicina proper the peristome is expanded, and not 

 vol. II. R R 



