PLATE XXXI. 
Page. 
Fig. 1 to 3. Strophomena septata W. and S 390 
1 and 2. Dorsal and profile views respectively of the same specimen. 
3. Interior of the ventral valve, to show the long median septum of this species. 
Trenton shales, St. Paul, Minn. Collection of C. Schuchert. 
Figs. 4 and 5. Strophomena rugosa (Raflnesque Ms.) Blainville 390 
4. Interior of the ventral valve. The septum of the muscular area does not extend 
to near the anterior margin, as in S. septata. 
5. Interior of the dorsal valve. Hudson River group, Oxford, Ohio. Collection of 
C. Schuchert. 
Figs. 6 and 7. Strophomena neglecta, var. acuta, n. var 388 
6. A view of the dorsal valve, to show the oblique wrinkling along the hinge line 
and distinct alternation of the stride. 
7 . Profile outline of the same specimen. Hudson River group, Spring Valley, Minn. 
Collection of C. Schuchert. 
Figs. 8 to 10. Strophomena planodorsata W. and S. 393 
8. A dorsal view of the largest specimen seen, somewhat restored. The outline of 
smaller individuals is more like that of fig. 10. 
9. Profile outline of same specimen. Hudson River group. Spring Valley, Minn. 
University of Minnesota collection. 
10. Interior of a ventral valve referred to this species. Hudson River group, Wilming- 
ton, Illinois. Collection of C. Schuchert. 
Fig. 11. Strophomena winchelli Hall 394 
A view of the dorsal valve of this species. (After Hall.) Lower Blue beds of 
Wisconsin. 
Figs. 12 and 13. Strophomena trilobat a 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. 
Figs. 14 to 17. Strophomena FLUCTUOSA Billings 395 
14 and 15. Dorsal and profile views of a specimen intended to show the strongly wrinkled 
visceral disc. 
16 and 17. The interior of a ventral and a dorsal valve respectively. Hudson River group. 
Spring Valley, Minn. Collection of C. Schuchert. 
Figs. 18 to 21. Strophomena scofieldi W. and S 398 
18 and 19. Dorsal and profile views of a specimen of the usual size of this species. 
20. An interior of the dorsal valve. Minnesota Geological Survey collection. 
21. Interior of the ventral valve. Galena shales, near Cannon Falls, Minn. Collec- 
tion of W. H. Scofleld. 
Figs. 22 to 24. Strophomena emaciata W. and S 399 
22 and 23. View of the ventral valve, natural size, and a profile of the same specimen, x 2. 
24. View from the dorsal valve, X 2. Galena shales, near Cannon Falls, Minn. Col- 
lection of W. H. Scofleld. 
Figs. 25 to 29. Rafinesquina mlnnesotensis N. H. Winchell 401 
25. 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. 
26. Profile view of a specimen from the Trenton shales, St. Paul, Minn. Collection 
of C. Schuchert. 
27 and 28. Variety in quassa Sardeson 403 
Dorsal and profile views respectively of a specimen of the ordinary size of this 
species as it occurs near the base of the Upper Buff beds at Mineral Point, Wis. 
Minnesota Geological Survey collection. 
29. Interior of the dorsal valve, showing the five characteristic septa of this species. 
(After Hall.) 
Figs. 30 and 31 . Rafinesquina 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. 
Figs. 32 to 34. Rafinesquina alternata (Conrad Ms.) Emmons, sp 404 
32 and 33. A ventral and a profile view of this species as it occurs in the Galena shales in 
Goodhue county, Minn. Minnesota Geological Survey collection. 
34. 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. 
Figs. 35 to 37. R. alternata, var. loxorhytis Meek, var 407 
35 and 36. 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. 
37. Interior of the dorsal valve. Hudson River group. Spring Valley, Minn. Min- 
nesota Geological Survey collection. 
