230 GASTROPODA. [Pectinibranchia. 



Genus 1. POTAMOPYRGUS, Stimpson, 1865. 



Potamopyrgus, Stimpson, Amer. Journ. Conch., i, 1865, 53. Type : Melania 

 corolla, Gould. Pyrgopkorus, Ancey, Bull. Soc. Mai. France, v, 1888. 

 188, 192. 



The animal has long, pointed tentacles ; the eyes on the prominent 

 tubercles. Stimpson's description of the dentition of the type is as 

 follows : ' Ehachidian tooth trapezoidal ; inferior margin nearly 

 straight, faintly trilobate ; basal teeth minute and close to the lateral 

 margins. Intermediate tooth with the peduncle very long, three 

 times as long as the body, and constricted at its juncture therewith ; 

 body subrhomboidal and excavated in the middle ; cusp with numerous 

 equal teeth. Lateral teeth constricted near the summit, and with tin- 

 dorsal or outer margin of the shank reflexed or thickened ; outer 

 lateral with a broad summit, shaped like a chopping-knife. Formula 



of the denticles : ^^- 11 - 15 - 20." (Stimpson's intermediate tooth 

 is really a lateral, and his laterals are the marginals.) 



Shell ovate-conic, imperforate ; apex acute ; whorls coronated 

 with spines ; outer whorl nearly two-thirds the length of the shell ; 

 aperture ovate ; outer lip acute ; operculum corneous. (Stimpson.) 



Many species are polymorphic, and one and the same species may 

 have a spinous angulate, an angulate espinous. and an acuminate 

 ecarinate form. 



The genus is, according to Pilsbry, of great antiquity, extending 

 at least as far back as the early Eocene. It now comprises all the 

 fresh-water Rissoids of New Zealand, a majority of those of Australia 

 and Tasmania, with species in West Africa and tropical America. 



Living in fresh and slightly brackish water. 



KEY TO SPECIES. 



A. The same species spinose. shouldered, or with smooth rounded 



whorls. 



a. Shell ventricose, last whorl swollen ; spines, if present, long, 

 directed upward. Ratio of diameter to height 1 : 1-44 

 to 1 : 1-7 . . . . . . corolla. 



aa. Shell smaller, much more slender, last whorl not ventri- 

 cose ; spines, if present, short. Ratio of diameter to 

 height 1 : 1-25 .. .. .. .. Badia. 



B. Shell never spinous or shouldered, smooth. 



a. Shell ovate. 



b. Height about 6mm., whorls flattish. outlines of spire 



straight . . . . . . . . antipodum. 



bb. Height less than 6 mm. 



c. Shell much elongated, height 4mm. to 5mm.. 



spire much higher than aperture . . . . egenus. 



cc. Shell shortly oval, height about 3 mm., spire li 



the height of aperture . . . . . . spelceus. 



aa. Shell subcylindrical, very thin and fragile . . . . sitbterraneus. 



