THE NATURAL HISTORY SURVEY. 



287 



b. Aperture expanded or bell-shaped. 



1. Both upper and lower surface forming a cone-shaped 

 depression, the edge of the last whorl, above and be- 

 low, bordered by a sharp carina bicarinatus 



2. Upper and lower surface on same plane, not form- 

 ing a depression; all of the whorls visible above and 



two rounded whorls visible below. No carina. ..campanulatus 



FIG. 95. 



Genitalia of PLANORBIS CORNEUS. (Bronn, Klassen und Ordnungen 

 der Weichthiere, Malacozoa, taf. CIII, Fig. 10.) dh, ovisperm duct; gh, 

 ovotestis; gal, albuminiparous gland; od, oviduct; p, penis; pr, prostate; 

 rs, receptaculum seminis. 



B. 4 Shell small, dextral. 



a. Periphery acutely keeled. 



1. Umbilicus narrow and rather deep exacutus 



b. Periphery rounded or obtusely keeled. 

 1. Umbilicus shallow and broad. 



| Whorls well rounded, aperture ovate .parvus 



ttWhorls angular, aperture deflected deflectus 



SUBGENUS HELISOMA Swainson, 1840. 

 "Shell ventricose, whorls angulated. Spire sunk below 

 the body whorl." (Dall.) 



