Melanopsis.] GASTROPODA. 237 



in front, very often with parietal plications ; operculum corneous, 

 spiral, with central or sublateral nucleus. 



The shell is very variable in form, and especially in the anterior 

 canal of the aperture, which is always rather short, but sometimes 

 disappears completely. 



The CerithiidcB occur principally about tropical and semitropical 

 shores, on rocks or among marine plants, littoral or in shallow waters. 

 Many of them are estuary molluscs, and some inhabit fresh waters. 



Subfam. 1. POTAMIDIN.E. 



Shells usually covered with a brown epidermis ; the fore part of 

 the aperture more or less channelled, truncate, not produced into a 

 beak ; operculum orbicular, polygyrate ; nucleus central. 



Inhabiting the mouth of rivers, or salt marshes. 



Genus 1. CERITHIDEA, Swainson, 1840. 



Cerithidea, Swainson, Man. Malac., 1840, 203, 342. Type : Murex decol- 

 latus, L. Aphanistylus, Fischer, Man. de Conch , 682, 1884. 



Animal with the eye-peduncles very long and thick, connate with 

 the tentacles nearly to their tips. 



Shell of moderate size, turriculated, conic ; spire varicose, very 

 often with truncated apex. Whorls narrow, convex, with a rather 

 deep suture, ornamented with axial costse, which are but little arcuate 

 and sometimes crossed by spiral cords. Height of aperture one- 

 third or one-fourth of the total length if the shell is not decollated. 

 Last whorl generally carinated or angled at the periphery of the base, 

 which is plane or oblique ; the axial ribs are not continued upon 

 the base. Neck very short or subobsolete, slightly excavated. Aper- 

 ture subcircular. Peristome reflexed, with a slight parietal channel. 

 Siphonal canal reduced to a lateral beak. Outer lip broadly rounded, 

 thickened. Columella smooth, neither twisted nor plicate, obliquely 

 truncate. 



These molluscs are amphibious, crawling on the stones and leaves 

 in the neighbourhood of brackish water in mangrove swamps, and 

 at the mouths of rivers ; during the dry season they close the aper- 

 ture with the operculum, and hang, suspended by glutinous threads, 

 to small branches and mangrove-roots. 



Fossil in the Tertiary. 



KEY TO SPECIES. 



A 



A Spire-whorls without spiral sculpture, except an occasional 



narrow groove below the suture . . . . . . sitbcarinata. 



B Spire-whorls with spiral sculpture. 



a. Deep spiral grooves, a narrow ridge in each . . . . alternnta. 



aa. Base with 2 spiral ridges, both near the periphery ; whorls 



spirally lirate . . . . . . . . . . bicarinttta. 



aaa. Base with 3 spiral ridges, spire-whorls with 24 narrow 



spiral grooves .. .. .. .. .. tricarinata. 



