MOLLUSCA OF MICHIGAN— WALKER. 



461 



(Biiiney.) 



Jaw arcuate, solid and strong, sculptured 

 with seven to twenty convex ribs; cutting edge 

 without median projection, but denticulated by 

 the ribs. 



Radula generally with tricuspid centrals and 

 bicuspid laterals and marginals, the side cusps 

 well developed; in some species, however, there 



are no side cusps or cutting points whatever on any of the teeth. 



Fig. 5. Dentition of P. albolabris. (Binney.) 

 KEY TO SECTIONS OF POLYGYRA. 



I. Aperture lunate, oblique, outer lip broadly expanded Triodopsis. 



II. Aperture narrow, basal, outer lip reflexed, not expanded beyond 

 contour of body whorl Stenotrema. 



Section TRIODOPSIS Rafinesque. 



Shell varjdng from depressed to globose conoidal, umbilicate or imper- 

 forate; surface generally striated; whorls 5-6, the last wider, more or less 

 defiexed in front; aperture lunate, typically obstructed by three teeth, two 

 on the lip, on the parital wall, but any or all of the teeth often absent. 



KEY TO THE SPECIES OF TRIODOPSIS. 



I. Aperture tridentate. 

 a. Striate and umbilicate. 



h. Peristome flattened, not inflected above, labial teeth small, of uni 



form size tridentata. 



66. Peristome strongly inflected above, aperture dish-shaped, 



upper labial tooth larger fraudulenta. 



aa. Hirsute and imperforate. 



6. Diameter exceeding 20 mill., lip broadly expanded, basal tooth long, 



lamellar palliata. 



66. Diameter less than 15 mill., lip not expanded beyond contour of 

 body whorl, basal tooth small and acute inflecta. 



