Figs. 1 to 5. 
land 2. 
a 
4. 
De 
Figs. 6 to 9. 
6and 7. 
8. 
9. 
Figs. 10 to 12. 
10. 
11. 
12. 
Figs. 13 to 17. 
13, 14 and 15. 
16 and 17. 
Figs. 18 to 23. 
18, 19 and 20. 
21. 
22. 
23. 
Figs, 24 to 30. 
24 and 25. 
26. 
27, 28 and 29. 
30. 
Figs. 31 to 34. 
31. 
32 and 33. 
34. 
Figs. 35 to 38. 
35 and 36. 
37 and 38. 
Figs. 389 to 42. 
39 and 40. 
41 and 42. 
42a. 
PLATEH XXXII. 
LEP TAN AC OMAR LOL Teal OWA GUS este eit nic settee cates lara vote te ele cicsleis sisvele ete e aisles 
Views of the ventral and dorsal valves respectively of an entire specimen, show- 
ing the zigzag surface ornamentation and the deflected anterior margin. 
Profile outline of the same specimen. 
Interior of the dorsal valve of a similar specimen. 
Interior of a ventral valve. 
logical Survey collection. 
Collection of C. Schuchert. 
Trenton shales, St. Paul, Minn, Minnesota Geo- 
LEPT#NA UNICOSTATA Meek and Worthen, Sp...-......0. secs seeeccee cece cecees 
An exterior view of the ventral valve and a profile outline of the same specimen. 
Minnesota Geological Survey collection. 
Interior of the ventral valve, showing the adductor and the elongate diductor 
muscle scars and the strongly tuberculose surface. 
Interior of the dorsal valve, which is much as in L. RHOMBOIDALIS, var. TENUT- 
STRIATA. Hudson River group, Spring Valley, Minn. Collection of C.Schuchert. 
PLECTAMBONUTES *SERICEAW SOW CLD Yay 8 Drreteteaversteterterctte erates attsycicte.steimene teierevelstersisrerslets 
A view from the dorsal valve of a specimen from the Hudson River group of Ohio. 
D, deltidium. (After Hall.) 
Interior of the dorsal valve of another specimen from the same locality. a, 
adductor muscle scars; ¢, crural plates; j, cardinal process; v, vascular impres- 
sions. (After Hall.) 
Interior of a ventral valve to show the crenulated hinge margin and the muscular 
scars. Hudson River group, Granger, Minn. Minnesota Geological Survey 
coliection. 
IPEEOTPAMBONTITES | GIBBOSA™ WNL Ce S sets ota siclevarotserniercpticloyotoleistaiavelelere centre ecreleremtie ee 
Ventral, dorsal and profile views respectively of an entire specimen, x 2. 
Dorsal and ventral valve interiors, drawn from gutta percha impressions made 
from natural casts. Middle Galena, near Cannon Falls, Mantorville and Old 
Concord, Minn. Minnesota Geological Survey collection. 
ORTHISPERICENARTAS COMPA Ce joaysetsereven clstareorere taiite siowian er nel wwinecs nesta oer 
Dorsal, ventral and profile views of a specimen wider than is common for the 
species. Minnesota Geological Survey collection. 
The interior of a well preserved dorsal valve. University of Minnesota collection. 
An interior of the ventral valve. E 
A cardinal view to show the small deltidia on both valves, and the chilidium of 
the dorsal valve, x 2. Trenton shales, Minneapolis, Minn. Collection of C. 
Schuchert. 
ORTHISE(DINOR THIS) DEREECTAY CONTAGs SPs weeccnies aidcabiemencebite «+s sisi cides cies 
Exterior and interior views of a ventral valve. (Fig. 24 after Hall.) 
The interior of a dorsal valve. (After Hall.) Trenton limestone, near Fountain, 
Minn. 
Ventral, dorsal and cardinal views of a rather thick specimen. (After Hall.) 
.Trenton limestone, Minneapolis, Minn. Collection of C. Schuchert. 
Profile outline of another specimen. Lower Blue beds, Janesville, Wis. 
sota Geological Survey collection. 
Minne- 
ORTHISy((DINORTHIS)) PECTINE LE Am MIN MONS neces. «1. sins oeeeiaesieccliiccecr =o 
The interior of the ventral valve of the type specimen of O. charlotte: Winchell. 
The outer margin of the shell is slightly broken. Trenton shales, Minneapolis, 
Minn. Minnesota Geological Survey collection. 
Dorsal valve and profile outline of another specimen. 
Cardinal view of the dorsal valve of a smaller specimen, to show the small del- 
tidium. Trenton shales, St. Paul, Minn. Collection of C. Schuchert. 
OF}(D)-PRCLINELEA, vary SWEBNEVI IN.yEls Winchell cw. . .sicuceacecmantce ets - 06 
View from the dorsal valve and profile of same specimen. (Fig. 35 after Hall.) 
Interiors of the dorsa] and ventral valves. (After Hall.) Trenton shales, St. 
Paul, Minn. Collection of C. Schuchert. 
ORTH (DINOR THIS) pMERDSL Wry ANG Suyrettelore otetatoreieloraler-icielsistster-il-iarenietetst-reeneeiciastee e 
Dorsal and cardinal views of a specimen in which the strie are separated more 
than is usual. 
Ventral and profile views of a larger specimen. 
Cardinal view. Collection of W. H. Scofield. 
Galena shales, Kenyon, Minn. 
411 
414 
416 
418 
424 
426 
