Fig. 1 to 3. 



1 and 2. 



3. 



Figs. 4 and 5. 

 4. 



Figs. 6 and 7. 

 6. 



Figs. 8 to -10. 



8. 



10. 

 Fig. 11. 



Figs. 12 and 13. 



Figs. 14 to 17. 

 14 and 15. 



16 and 17. 



Figs. 18 to 21. 



18 and 19. 



20. 



21. 



Figs. 22 to 24. 



22 and 23. 



24. 



Figs. 25 to 29. 

 25. 



26. 



27 and 28. 



29. 

 Figs. 30 and 31. 



Figs. 32 to 34. 

 32 and 33. 



34. 



Figs. 35 to 37. 

 35 and 36. 



37. 



PLATE XXXI. 



STKOPHOMEN A SEITATA W. and S 



Dorsal and profile views respectively of the same specimen. 

 Interior of the ventral valve, to show the long median septum of this species. 

 Trenton shales, St. Paul, Minn. Collection of C. Schuchert. 



STROPHOMBNA BUQOSA (Raflnesque Ms.) Blainville 



Interior of the ventral valve. The septum of the muscular area does not extend 



PAGE. 



390 



to near the anterior margin, as in S. septata. 

 Interior of the dorsal valve. Hudson River group, Oxford. Ohio. 

 0. Schuchert. 



Collection of 



STROPHOMENA NEGLECTA, var. ACUTA, n. var 



A view of the dorsal valve, to show the oblique wrinkling along the hinge line 



and distinct alternation of the striae. 

 Profile outline of the same specimen. Hudson River group, Spring Valley, Minn. 



Collection of C. Schuchert. 



STROPHOMENA PLANODOPS ATA W. and S 



A dorsal view of the largest specimen seen, somewhat restored. The outline of 

 smaller individuals is more like that of fig. 10. 



Profile outline of same specimen. Hudson River group, Spring Valley, Minn. 

 University of Minnesota collection. 



Interior of a ventral valve referred to this species. Hudson River group, Wilming- 

 ton, Illinois. Collection of C. Schuchert. 



STROFHOMENA WINCHELLJ Hall 



A view of the dorsal valve of this species. 

 Wisconsin. 



390 



388 



393 



(After Hall.) Lower Blue beds of 



394 



STROPHOMENA TRILOBATA Owen 395 



Dorsal and profile views of a specimen showing the small, fiat corrugated visceral 

 disc, and the long median extension. Middle Galena, Old Concord, Minn. Col- 

 lection of C. Schuchert. 



STROPHOMENA FLUCTUOSA Billings 395 



Dorsal and profile views of a specimen intended to show the strongly wrinkled 



visceral disc. 



The interior of a ventral and a dorsal valve respectively. Hudson River group, 

 Spring Valley, Minn. Collection of C. Schuchert. 



STROPHOMENA SCOFIELDI W. and S 



Dorsal and profile views of a specimen of the usual size of this species. 

 An interior of the dorsal valve. Minnesota Geological Survey collection. 

 Interior of the ventral valve. Galena shales, near Cannon Falls, Minn. Collec- 

 tion of W. H. Scofleld. 



398 



399 



STROPHOMBNA EMACIATA W. and S 



View of the ventral valve, natural size, and a profile of the same specimen, x 2. 

 View from the dorsal valve, x 2. Galena shales, near Cannon Falls, Minn. Col- 

 lection of W. H. Scofleld. 



RAFINESQUINA MINNESOTENSIS N. H. Winchell 401 



View of a natural cast, one of the type specimens, of the ventral valve as pre- 

 served in the Trenton limestone, Minneapolis, Minn. Minnesota Geological 

 Survey collection. 



Profile view of a specimen from the Trenton shales. St. Paul, Minn. Collection 



of C. Schuchert. 

 Variety INQUASSA Sardeson 403 



Dorsal and profile views respectively of a specimen of th'e ordinary size of this 

 species as it occurs near the base of the Upper Buff beds at Mineral Point, Wis. 

 Minnesota Geological Survey collection. 



Interior of the dorsal valve, showing the five characteristic septa of this species. 

 (After Hall.) 



RAFINESQOINA DELTOIDEA Conrad, sp 403 



Views of a natural cast of the interior of the ventral valve and a profile of same. 

 Middle Galena, Mantorville, Minn. Minnesota Geological Survey collection. 



RAFINESQUINA ALTERNATA (Conrad Ms.) Emmons, sp 404 



A ventral and a profile view of this species as it occurs in the Galena shales in 



Goodhue county, Minn. Minnesota Geological Survey collection. 

 Interior view of a thick dorsal vavle in which the various parts are strongly 



developed. Trenton shales, St. Paul, Minn. Collection of W. H. Scofleld. 



R. ALTERNATA, var. LOXORHYTIS Meek, var 407 



View of the ventral valve and a profile outline of the same specimen, to show the 



great convexity and transversity of this variety. Collection of C. Schuchert. 

 Interior of the dorsal valve. Hudson River group, Spring Valley, Minn. Min- 

 nesota Geological Survey collection. 



