534 MOLLUSCA 



Family 3. NERITIDAE. 



Shell thick, semi-globose, with a small spire and a very large lower 

 whorl and aperture; operculum calcareous; cavity simple in consequence 

 of the absorption of the internal portions of the whorls; animal with a 

 broad, short snout; tentacles slender; eyes stalked ; epipodium not fringed: 

 many hundred species, mostly marine, a few in fresh water and terrestrial. 



Nerita L. Shell smooth or spirally ridged; outer lip thick, usually 

 with teeth within : over 200 species, which live on rocks, feeding on algae. 



N. fulgurans Gmelin. Shell Avith spiral ridges, black or gray in 

 color; aperture white, with prominent teeth; 18 mm. long; 30 mm. wide: 

 West Indies; California. 



Family 4. HELICINIDAE. 



Shell conical or depressed, heliciform, with a single cavity due to 

 the absorption of the inner portions of the whorls; columella generally 

 with a thick callus, which covers the umbilicus; lip thick; operculum 

 oval or triangular; tentacles slender, with eyes at their base; ctenidia 

 absent, the mantle cavity being transformed into a lung: over 500 

 species, all terrestrial, chiefly in warm countries; about 4 species in the 

 southern states (70 species in Cuba). 



Helicina Lamarck. With the characters of the family: about 350 

 species in tropical America and the Pacific islands. 



H. orbiculata Say. Shell yellowish in color, sometimes spotted, 

 with 5 whorls; 6 mm. high; 9 mm. in diameter: southern states, south 

 of Tennessee. 



Suborder 2. PECTINIBRANCHIATA. (Monotocardia; 

 Ctenobranchiata. ) 

 But one auricle, kidney, osphradiura, and ctenidium present, the cteni- 

 dium being monopectinate or feathered on but one side (Fig. 821, C) and 

 attached to the mantle its whole length (except in Janthina and Valvata) ; 

 proboscis, siphon, anterior or siphonal canal, and penis usually present; 

 gonad never opens into the kidney but has a separate duct : over 50 fami- 

 lies, grouped in 5 divisions and comprising most of the prosobranchs. 



Key to the divisions of Pectinihranchiata : 

 a^ Radula short and wide, with numerous lateral and usually no central 



teeth ; shell without siphonal canal 1. Ptenoglossa 



a, Radula long and narrow, or wanting ; siphonal canal either present or 

 absent. 

 6i Radula with 7 teeth in each transverse row (Fig. 830) ; canal present 



or not ; the majority of species 2. T^nioglossa 



6j Radula and jaw wanting; shell elongate, without canal... 3. Gymnoglossa 



63 Radula with 3 teeth in each row (Fig. S70) ; shell thick, conical, with 



canal 4. Rachiglossa 



64 Radula with 1 or 3 teeth in each row ; shell with an elongate aperture 



and a canal 5. Toxoglossa 



