238 SNAILS AND SLUGS [CH. VII 



stagnalis (L.), 2", whorls 7 or 8, last enlarged suddenly, suture deep. 

 truncatula Mull, f ", whorls 5 or 6, spire long, suture very deep, 



umbilical fissure distinct, 

 genus Amphipeplea Nilsson, glutinosa (Mull), ^", spire very short, 



whorls 3 or 4, last forming nearly whole shell, suture shallow, 



aperture oval. Shell almost entirely covered by mantle, very 



thin, transparent. 



Family PHYSID^E. Shell sinistral, lustrous, thin. Tentacles cylin- 

 drical, setaceous, not retractile. No operculum. 

 genus Aplecta Flem. em., hypnorum (L.), f", shell spiral, oval, 



very convex, apex blunt ; umbilicus a narrow cleft ; aperture 



simple, sharp-edged. Mantle devoid of finger- like projections 



over the shell. Whorls 6 or 7, spire long, 

 genus Physa Drap., fontinalis (L.), ", shell spiral, oval, oblong, 



transparent ; aperture vertical, elongate, simple, sharp-edged. 



Mantle with finger-like prolongations over the shell, whorls 



4 or 5, spire short. 

 The species Ph. aouta Drap. and Ph. heterostropha Say. are 



introduced. 



Sub-family Planorbince. 

 genus Planorbis Geoff., shell a flat coil, dextral; aperture cre- 



scentic ; tentacles cylindrical. 

 fontanus (Lightfoot), ^", whorls 4 or 5 ; thin, shining, compressed, 



keeled in middle, last whorl not embracing the rest. 



(a) whorls rounded. 

 albus Miill., j", whorls 5 ; grayish white, with longitudinal striae, 



last whorl large. 



contortus (L.), \" whorls 8, compact, shell thick ; aperture semi- 

 lunar ; umbilicus large, deep. 

 corneus (L.), 1", whorls 5 or 6, rounded above and below ; suture 



deep ; aperture narrow, crescentic, umbilicus large, deep. 

 glaber Jeff., ^", whorls 5, last whorl large, shell convex above, 



concave below, smooth, no circular striae ; umbilicus large. 



(6) whorls keeled or angulated. 

 carinatus Miill., |", whorls 5 or 6 ; keel in middle of whorls, last 



whorl large, aperture obliquely oval. 

 umbilicatus Miill., f" whorls 5 or 6 ; keel on lower side of whorls, 



last whorl large ; aperture rhombic. 



