PLATE XXXIII 
Paok. 
Pigs. ] to 4. Oktuis (ITebektella?) BEr.LARUGOsA Conrad 434 
1 and 2. Views of the exterior and interior of a large ventral valve. Galena shales, 
Minneola, Minn. Minnesota Geological Survey collection. 
3 and 4. Exterior and interior views of a dorsal valve of the ordinary size. Galena 
limestone near Fountain, Minn. Collection of C. Schuchert. 
Figs. 5 to 7. Oktjtis (PLECTOiirnis) plicatella Ilall 4:k; 
Ventral, dorsal and profile views respectively of an entire specimen. Galena 
shales, K(!nyon, Minn. Collection of C. Schuchert. 
Figs. 8 to 13. Ohtuis (Plectouthis) wniTFTELDi N. n. Winchell 437 
8 and 9. Ventral and dorsal views of one of the type specimens, a natural cast of the 
interior. 
10 and 11. A view of the ventral valve and profile outline of same of another type speci- 
men. Hudson River group, Spring Valley, Minn. Minnesota Geological 
Survey collection. 
12. The interior of a small ventral valve. 
13. A large ventral valve, abnormal in its growth. Hudson River group, Spring 
Valley, Minn. Minnesota Geological Survey collection. 
Figs. 14 to 16. Orthis (Dalmanella) hamburgensis? Walcott 440 
14 and 16. Ventral, profile and dorsal views respectively of an entire specimen referred 
with doubt to this species; 16a, fig. 16 x 2. Trenton shales near Fountain, 
Minn. Minnesota Geological Survey collection. 
Figs. 17 to 22. Okthis (Dalmanella) testudinaria Dalman 441 
17 and 18. Ventral and dorsal views of a specimen of this species from the Trenton 
shales, Minneapolis, Minn.; flig. 18X2. 
19 and 20. Dorsal and profile outlines of the same specimen, x 2, Minnesota Geological 
Survey collection. 
21 and 22. Interior of the ventral and dorsal valves, x 2. St. Paul, Minn. Collection of 
C. Schuchert. 
Figs. 23 and 24. O. (D.) testudinaria var. emacerata Hall 445 
Two views, a dorsal and profile, of a specimen referred to this variety. Hud- 
son River group, Spring Valley, Minn. Minnesota Geological Survey collection. 
Figs. 25 to 29. O. (D.) testudinaria var. meeki Miller 445 
25 to 27. Ventral, dorsal and profile views respectively of an entire specimen. 
28. Profile view of another somewhat thinner specimen. Minnesota Geological Sur- 
vey collection. 
29. The interior of a dorsal valve in which the parts are strongly developed. 
Hudson River group near Spring Valley, Minn. Collection of C. Schuchert. 
Figs. 30 to .36. Orthis (Dalmanella) sub^quata Conrad 446 
30 to 32. Ventral, dorsal and profile views respectively of the type specimen of Orthis 
/umneapoh's Winchell. Trenton shales, Minneapolis, Minn. Minnesota Geo- 
logical Survey collection. 
33, 34 and 34a. Dorsal, ventral and profile views of a specimen in which the fold and sinus are 
obsolete and the tubular strite conspicuous. Trenton shales, Minneapolis, 
Minn. Collection of C. Schuchert. 
35. Interior of the ventral valve, x 2. (After Hall.) 
36. Interior of the dorsal valve. (After Hall.) Base of the Upper Buff limestone, 
Mineral Point, Wisconsin. 
Figs. 37 to 39. O. (D.) sub^quata var. conkadi N. H. Winchell 449 
37. View from the dorsal valve of a typical example of this variety, x 2. 
38 and 39. Ventral and profile views of the same specimen of the natural size. Trenton 
limestone, Minneapolis, Minn. Minnesota Geological Survey collection. 
Figs. 40 to 42. O. (D.) suBiEQUATA var. PERVETA Conrad 450 
40 and 41. Ventral and dorsal vievys of the type specimen of Orthis media Winchell. Base 
of Trenton shales, Minneapolis, Minn. Minnesota Geological Survey collection. 
42. View from the dorsal valve of Conrad's type specimen of Orthis perveta. (After 
Hall.) Base of Upper Buff beds at Mineral Point, Wisconsin. 
Figs. 43 to 45. O. (D.) sub^equata var. oibbosa Billings 451 
Dorsal, ventral and anterior views of a characteristic specimen of this variety. 
Trenton shales, Minneapolis, Minn. Collection of C Schuchert. 
