PLATE 61 a. 
Page 
Figs. 1 - 21, 25, 26. Centronella glans-fagea. .. 399 
1, 2. Dorsal and venfral views of a large specimen of broader form than usual. 
3-5. Dorsal, ventral and profile views; enlarged to three diameters, from a specimen of ordi¬ 
nary form. This, and the preceding specimen, are from the Schoharie grit. 
6. Dorsal view of a large individual, from the Upper Helderberg limestone. 
7. Dorsal view of a specimen from a drift pebble of limestone near Ann Arbor, Michigan, 
which has the dorsal valve entirely flat; enlarged to two diameters. 
8-10. Dorsal, ventral and profile views of a large specimen from the Onondaga limestone of 
Erie county, N. Y.; enlarged to two diameters. 
11 - 13. Dorsal, ventral and profile views of a very gibbous form, which shows the foramen and 
deltidium, enlarged to two diameters. The specimen is from the drift of Michigan. 
14, 18. Dorsal and ventral views of a small specimen of somewhat different form, from the 
same locality as the preceding; enlarged to two diameters. 
16. The interior of a dorsal valve, showing the muscular impressions, cardinal processes, the 
points for the attachment of the loop, and the dental sockets; enlarged to three diameters. 
17. Dorsal view of a cast of a large specimen from the Schoharie grit, showing the muscular 
impressions, etc.; enlarged to two diameters. 
18. The cast of a ventral valve from the Schoharie grit; enlarged to two diameters. 
19. Dorsal view of a cast in chert from the drift of Michigan, showing the muscular imprints, 
the filling of the rostral cavity of the ventral valve retaining the imprints of the delti- 
dial plates. 
20. Enlargement of the loop of this species, taken from the ventral side showing its 
attachment to the hinge-plate of the dorsal valve. 
21. Profile of the same. 
25, 26. Dorsal and profile views of a very ventricose specimen having the character of C. 
tuviida. Billings, but of smaller size. The specimen is from the Upper Helderberg 
limestone of Cayuga, C. W., and the figures are enlarged to two diameters. 
Figs. 22-24. Centronella alveata . 401 
22 - 24. Dorsal, ventral and profile views of the specimen described. 
Figs. 27-29. Centronella hecate.... 420 
27-29. Dorsal, ventral and profile views of a specimen of this species from Cayuga, C. W.; 
enlarged to two diameters. 
Figs. 30-38. Centronella impressa.•. 402 
30-32. Dorsal, ventral and profile views of a young and somewhat circular form. 
33-35. Dorsal, ventral and profile views of a full grown specimen of the ordinary size. 
36. Interior of a dorsal valve, showing the cardinal process and the strong callosities to which the 
crura are attached. 
37. Interior of a dorsal valve of circular form, showing muscular and vascular markings; en¬ 
larged to two diameters. 
38. Interior of a ventral valve, showing the teeth and deltidial plates. 
Figs. 39,40. Centronella glatjcia... 403 
39. Partial cast of a large dorsal valve. 
40. Cast of a ventral valve, showing the muscular impression and filling of the rostral cavity. 
Figs. 41-45. • Centronella julia . 419 
41, 42, 43. Dorsal, ventral and profile views of a specimen of this species; enlarged to two 
diameters. 
44. Yentral view of a large broad form; enlarged to two diameters 
45, 46. Yentral and profile views of the loop of this species, made from specimens which have 
been broken, so as to show parts of the structure in different individuals from which this 
has been restored. Specimens loaned by Prof. A. Winchell. 
Figs. 47 - 49. Centronella ovata. 419 
47 - 49. Dorsal, ventral and profile views of the specimen described. 
Figs. 50-52 Tropidoleptus occidens. 408 
50-52. Dorsal, ventral and profile views of a specimen of ordinary size. 
