546 MOLLUSCA 



with an expanded outer lip; operculum horny and spiral; mantle with 

 a short siphon : 200 species, in salt, brackish, and fresh water. 

 Key to the genera of Cerithiidae here described: 



(^ Shell dextral. 



&i Outer lip not expanded 1. BITTIUM 



63 Outer lip expanded 3. CEEITHIOPSIS 



a, Shell sinistral 2. TBIFOBIB 



1. BITTIUM Leach. Shell with a granular surface; aperture with 

 a slight canal in front; outer lip acute, not expanded; operculum with 

 4 spirals : numerous species. 



B. alternatum (Say) (B. nigrum Stimpson) (Fig. 

 851). Shell slender, with about 8 whorls, bluish or blackish 

 in color, 6 mm. long and 2 mm. wide ; surface covered with 

 a fine network of elevated lines ; aperture rounded : Massa- 

 chusetts Bay to New Jersey; in shallow water. 



2. TRIFORIS Deshayes. Shell sinistral; aperture 

 round, prolonged in front to form a closed tube: 100 

 species, mostly towards the south. 



T. perversa (L.) (T. nigrocincta Adams). Shell reddish-black in 

 color, with 3 revolving series of granules; whorls 12 or more; length 7 

 mm.; width 1.7 mm.: Cape Cod to west Florida; rather 

 common; Europe; California. 



3. CERITHIOPSIS Forbes and Hanley. Shell slender, 

 frequently with longitudinal ribs; aperture with an an- 

 terior canal and a less distinct posterior one; operculum 

 horny and spiral: in shallow water. 

 Fig. 852 0. greeni (Adams) (Fig. 852). Shell with 10 to 12 



greeni whorls, reddish-black in color, with longitudinal ridges 



and revolving lines, forming series of granules; length 5 

 mm.; width 1.2 mm.: Massachusetts Bay to Texas. 

 C. terebralis (Adams). Shell with 10 to 12 whorls, flattened and 

 having 3 revolving ridges on each, with numerous fine longitudinal 

 lines between the ridges; length 12 mm.; width 3 mm.; color reddish- 

 brown: Cape Cod to Texas. 



FAMILY 11. PLEUROCEBIDAE. 



Shell elongate, conical, with numerous whorls; operculum spiral; 

 aperture round or ovate, and rather small and channeled or angulated 

 in front; animal with a broad, short snout and with eyes sessile, 

 situated at the base of the tentacles: in fresh water; many hundred 

 species, all in the United States (except 2 or 3 in Mexico), mostly in 

 the southern states. 



