PLATE 61 A. 



Pago 



Figs. 1-21, 25, 26. Centronklla olans-fauea 399 



1, 2. Dorsal and ventral views of a large specimen of broader form than usual. 



8-5. Uorsiil, ventral and profile views; enlarged to three diameters, from a specimen of ordi- 

 nary form. This, and the preceding specimen, are I'roni the Schoharie grit. 



6. Dorsal view of a large individual, from the Upper llelderl)erg limestone. 



7* Dorsal view of a specimen from a drill pebble of limestone near Aim Arbor, Michigan, 

 which has the dorsal valve entirely Hat ; enlarged to two diameters. 



8-10. Dorsal, ventral and protile views of a large specimen from the Onondaga limestone of 

 Erie county, N. Y.; enlarged to two diameters. 



11 - 13. Dorsal, ventral aud profile views of a very gibbons form, which shows tlie foramen and 

 deltidium, enlarged to two diameters. The specimen is from the drift of Michigan. 



14, 16. 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, shovJlng the muscular inijircssions, cardinal processes, the 



points for the attachment of the loop, and the dental sockets ; enlarged to tliree diameters. 



17. Dorsal view of a cast of a large specimen from tlie 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 



attaelmieut to the hinge-plate of the dorsal valve. 



21. Protile of the same. 



26, 26. Dorsal and protile views of a very ventricose specimen having the character of C 

 tumidu, 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. 

 86. 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 glaucia 403 



89. Partial cast of a large dorsal valve. 



40. Cast of a ventral valve, showiug 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. Ventral view of a large broad form ; enlarged to two diameters 

 46, 46. Ventral and profile views of the loop of this species, made from specimens which have 



been broken, so as to show parts of the structure iu difi'erent 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 



60-62. Dorsal, ventral and profile views of a specimen of ordinary size. 



