CONCHOLOGT. 119 



3. Pirena. Four species. 

 This shell resembles the Melania, but is easily distinguish- 

 ed by having a sinus at the base and another at the summit. 

 Shell turreted, aperture oblong, closed by a horny opercu- 

 lum ; right lip sharp, with a distinct sinus at the base and 

 another at the summit ; base of the columella inclined to- 

 wards the right. 



Pirena terebralis. Pirena aurita, 



P. spinosa. P. granulosa. 



P. terebralis. The wimble Pirena. 

 Subulate, longitudinally striated, covered with a dark- 

 brown epidermis ; aperture white, outer lip expanded. 



FAMILY X. 



Peristomides. Three genera. 



1. Valvata. One species. 



This genus contains shells found in fresh water. 



Shell sub-discoid or conoid, umbilicated, spiral whorls 

 rounded ; summit papillose ; aperture round, not modified by 

 the last whorl ; the edges completely united, sharp ; opercu- 

 lum complete, horny, and orbicular. 



V. piscinalis. The Pond Valvata. PI. 20, fig. 5. ' 

 Small, globular, conoid, deeply umbilicated ; summit ob- 

 tuse ; wrinkled longitudinally, covered with a yellowish epi- 

 dermis. 



2. Paludina. Seven species. 



Generally inhabits fresh water, though some have been 

 found where it is quite saline. 



Shell conoid, covered with epidermis, spiral whorls round- 

 ed ; rather longer than broad, edges united, always sharp; 

 the commencement of the left edge immediately attached to 

 the last whorl of the spire ; operculum horny, complete, or 

 marginal, not spiral, with concentral elements. 



