EXPLANATION^ TO PLATE 1. 



Representative individuals of the forms of the genus lo, showing types, typical specimens, 

 and a diagram of the upper Tennessee River to show the general range of each form in the sys- 

 tem. The numbei-s under each kind of shell correspond with the numbers and range of the 

 shell as shown on the map above. The shells in the upper row probably begin development 

 smooth; those in the lower row as undulate or spinose. 



1. poweUensis. 9. nolichucJcyensis. 



2. clincJiensis. lO. recta. 



Z. fiuvidlis. 11. spinosa. 



4. lyttonensis. 12. angitremoides. 



5. paulensis. 13. loudonensis (ranges downstream 



6. verrucosa. to Bridgeport, Ala.). 



7. hrevis. 14. turrita (ranges downstream to 



8. unakensis. the Muscle Shoals, Ala.). 



X = spinose undetermined shells. Natural size. 

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