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Cincinnati Society of Natural History. 



EXPLANATION OF PLATE V. 



A. — Segmentina armigera Say. c. Central tooth. I, First lateral. 

 10, Transition tooth, n, 15, Marginal teeth. 



B. — Ancylus rivularis Say. c. Central tooth. 1, First lateral. 5, 

 Transition tooth. 9, 13, Marginal teeth. 



C. — Physa heterostropha Say. c. Central tooth. 1, Principal tooth. 

 2, Secondary tooth. 3, Jaw. 



D. — Aplexa hypnorum Linne. c. Central tooth (cusp). 1, Lateral 

 tooth. 



E. — Pleurocera elevatum Say. c. Central tooth. 1, Intermediate 

 tooth. 2, 3, Lateral teeth. 



F. — Elimnia livescens Menke. c. Central tooth. 1, Intermediate 

 tooth. 2, 3, Lateral teeth. 4, Jaw. 



All but C, 3 and F, 4 X 600. 



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