Nerita.] GASTROPODA. 171 



oblong, wide in front, attenuated behind ; branchia long, triangular, 

 pointed, free at the extremity, ventricle embracing the intestine, 

 anus on the right side ; a cephalic male organ present. The formula 

 of the teeth of the radula is oo 1 . (3 + 1 + 3) . 1 oo ; the central tooth 

 small, subquadrangular ; second central tooth very large, transverse, 

 subrhomboidal ; third and fourth central teeth very small ; lateral 

 tooth with reflected, simple or denticulated margin ; marginal teeth 

 numerous, narrow, curved, serrated. 



Shell imperforate, thick, semiglobose, porcellanous ; spire very 

 small, internally porcellanous, the cavity simple from the absorp- 

 tion of the internal portion of the whorls by the animal. Aperture 

 semilunate, entire, the columellar lip flattened, septiform, with a 

 rectilinear, plain, or dentate margin ; outer lip rounded, sharp, or 

 thickened, not reflected. 



Operculum calcareous, usually subspiral, provided with projecting 

 lobes on its inner face, the inner margin forming a pseudo-articulation 

 with the columellar lip. 



These molluscs are aquatic, although some species can normally 

 live out of water. They are herbivorous, and deposit their eggs on 

 their own shell or on that of another animal. The eggs are round 

 or oval, with a thick, coriaceous, subcalcareous envelope, which sepa- 

 rates transversally when the embryo emerges, the upper part of the 

 capsule remaining attached to the lower part like a capsule. 



Trias to Recent ; living in tropical and warmer seas. 



Genus 1. NERITA (L.), Lamarck, 1799. 



Xerita, Linne, Syst. Nat., eci. x, 1758. 77fi : Lamarck, Mem. Soc. H.X., 

 Paris. Type : N. albicilla, L. Nerita, Lam. : Man. Conch. (1), x, 4, 18. 



Foot without anterior marginal groove, epipodial line consisting 

 of a simple membranaceous duplication extending from the tentacles 

 to the operculum ; mantle-margin usually festooned. 



Shell thick, smooth, or spirally ridged and grooved, porcellanous, 

 under a corneous adhering epidermis sometimes wanting ; outer lip 

 thick, usually denticulated within ; columellar lip flattened, its margin 

 dentate, straight. Operculum calcareous, the outer face granulated 

 or with a decurrent groove, paucispiral. with excentric nucleus ; inner 

 face callous, the apical and claviform apophyses well marked, marginal 

 apophysis more or less developed, corresponding with tne groove or 

 the outer face. 



Animals living on rocks and stones, generally inactive by day, 

 but said to be active at night, roaming about and feeding on alga?, &c. 

 Gregarious, and littoral, and nearly exclusively marine. 



About 200 species have been described, inhabiting tropical and 

 semitropical shores throughout the world. 



Fossil. Cretaceous and Tertiary. The earlier forms are sorae- 

 what doubtful, and belong very likely to the genus Neridomus, M. & L. 



