350 The Molluscan Family Planorbidae 



PLATE 57 



Radulae of Helisoma 



See remarks on plate 53. Line at bottom of plate indicates 25 microns in length. 



L Helisoma truncatum (Miles). Winnebago Lake, near Oshkosh, Wisconsin. 



Normal membrane. 85. center tooth; 1-7, lateral teeth; 9-11, intermediate 

 teeth; 13-20, marginal teeth. 



2. Helisoma truncatum (Miles). Winnebago Lake, Wisconsin. 



Abnormal membrane, with many split cusps on teeth. 90, center tooth; 1-8, 

 lateral teeth, 6 and 7 with bifid entoconic cusps and 5, 6, and 7 with split 

 ectoconic cusps; 9-11, intermediate teeth with modified ectocones; 12-21, 

 marginal teeth. 



3. Helisoma corpulent um (Say). Lac la Croix, Ontario, Canada. 



48, center tooth; 1-8, lateral teeth; 9-12, intermediate teeth; 13-25, marginal 

 teeth. 



4. Helisoma magnijicum (Pilsbry). Near Wilmington, North Carolina. 



90, center tooth; 1-14, lateral teeth; 16-23, intermediate teeth; 24-41, marginal 

 teeth. 



The small numerals on the first lateral of fig. 1 indicate the following parts 

 of the tooth: 1, entocone or inner cusp; 2, mesocone or middle cusp; 3, 

 ectocone or outer cusp; 4, base of attachment to radula membrane. In 16 

 of fig. 1, the small numeral 2 indicates the position of the mesocone in a 

 marginal tooth. 



