MOLLUSCA. GASTEROPODA 



243 



Viviparus { = P<dudina). Shell thin, turbinate, with a thick 

 periostracum ; whorls convex, smooth or with faint ribs. Umbilicus 

 small or absent. Aperture entire, oval, slightly angular behind. 

 Operculum horny with concentric striae, and excentric nucleus. 

 Inferior Oolite to present day. Lives in fresh water. Ex. V. hntus, 

 Bembridge Beds. 



Turritella. Shell without umbilicus, turreted, with many 

 flat or slightly convex whorls, ornamented with spiral ribs and 

 with stria? of growth ; spire very long and acute. Aperture oval 

 or sub-quadrate, entire, outer lip thin, slightlj 7 produced in front. 

 Operculum horny. Trias to present day. Ex. T. communis, 

 Pliocene to present day ; T. imbrieataria, Barton and Bracklesham 

 Beds. 



Melania. Shell with dark periostracum, elongate, turreted, 

 with many whorls, without umbilicus, 

 apex sharp but usually corroded. Sur- 

 face smooth, or ornamented with spiral 

 striae, or transverse ribs, or spines. 

 Aperture entire, narrow behind, rounded 

 in front ; outer lip sharp, slightly sinu- 

 ous behind. Columella smooth. Oper- 

 culum horny, oval, sub-spiral. Wealden 

 to present day. Lives in fresh water. 

 Ex. M. amanda, Recent ; M. acuta, 

 Bembridge Beds. 



Nerinea (fig. 100). Shell elongate, 

 usually without an umbilicus, whorls 

 numerous. Aperture sub-quadrangular, 

 oval, or elongate, with a short anterior 

 canal ; outer lip thin, with a posterior 

 slit near the suture, which becomes 

 filled, leaving a continuous band. Colu- 

 mella and also the interior of the 

 whorls furnished with folds which are 

 continuous to the apex. Inferor Oolite 

 to Upper Cretaceous. Ex. N. cingenda, 

 Inferior Oolite. 



Cerithium. Shell without an um- 



Fig. 100 



Nerinea trachea, 

 partly sliced to show 

 the form of the in- 

 terior. Great Oolite. 

 (From Woodward.) x f . 



16—2 



