788 DIVISIONS OF THE GASTROPODA. 



The principal or sole genus in this family is Nerinea itself, 

 which has been divided into several sub-genera. In this genus 

 the shell (figs. 691 and 692) is turreted, many-whorled, and nearly 

 cylindrical. The columella carries continuous ridges, and similar 

 ridges exist on the interior of the whorls, so that casts of the 

 interior are often very unlike the exterior. The genus Nerinea 

 is wholly Jurassic and Cretaceous, a very large number of species 

 being found in the former of these systems, and particularly in 

 strata of the age of the Coral Rag. 



Family 33. Cerithiid^e. — In this family the shell (fig. 693) is 

 spiral and turreted. The aperture is oval or quadrate, and is 

 channelled in front, the canal being short and often bent back- 

 wards. The outer lip is generally expanded in the adult, the 

 columella is sometimes thickened, and the operculum is horny and 

 spiral. The members of this family are marine or live in brackish 

 waters, some forms frequenting salt marshes or the mouths of rivers. 

 More than three hundred recent species, and over a thousand fossil 

 forms, of this family have been described, the most ancient types 

 appearing in the Trias. The known forms belong to the two 

 comprehensive genera Cerithium and Potamides, each of which 

 has been split up into minor groups, to some of which a generic 

 value is sometimes attached. 



In Cerithium proper (fig. 693) the shell is without an epidermis, 

 the operculum is paucispiral, and the aperture has 

 a well-developed, backwardly-bent canal. The 

 species of Cerithium are essentially marine, and 

 the genus has been broken up into sub-generic 

 groups {Fibula, Eustoma, &c), the distinctive 

 characters of which are mostly of small import- 

 ance. The species of Cerithium abound in the 

 Secondary and Tertiary rocks, and in warm seas 

 at the present day. The oldest forms appear in 

 the Alpine Trias. 



The genus Potamides closely resembles Ceri- 

 thium in general characters ; but the surface is 

 covered with epidermis, the aperture has only a 

 F ig. eg?,.— Cerithium short canal or a notch in front, and the operculum 

 TeS~ E ° cene is multispiral. The species of Potamides live in 

 brackish waters or at the mouths of rivers. In 

 the fossil condition the shells of Potamides cannot be certainly 

 distinguished from those of Cerithiiwi, but it is usual to consider 

 that the representatives of this family which occur in estuarine or 

 fresh-water deposits are referable to the former genus, while those 

 which occur in purely marine deposits belong to the latter. On this 

 view, the species of Potamides are Cretaceous, Tertiary, and Recent. 



