PLATE LIII. 
(Figures 1-3, 5-7, 11-13,17-23, 29,30, 36-38, 46, 48, 54. 55 by G. 15. Simpson; 4 by J. M. Clarke; 8, 9,14, 15, 31, 40-42, 45, 
47, 49, 51, 53 by E. Emmons; 10,16, 24-28, 32-35, 39 by K. P. Whiteield; 43, 44, 50, 52 by F. 15. Meek ) 
Genus ATRYPTNA, gen. nov. 
Page 161. 
AtRYPINA mSPARILIS, Hull. 
Fig'S. 1-3. Ventral, profile and dorsal views of an average niatuie exainjile ; showing the plano-convex 
contour, sparse plication and concentric surface markings. X 2. 
Fig. 4. A dorsal view of a vei'y young individual; showing the deltidial plates beneath the erect beak, 
the deep median sinus of the brachial valve and its low plications. X 5. 
Niagara gi'oup. Waldron, Indiana. 
Atrypina imbricata, Hall, 
Fig. 5. The interior of a pedicle-valve ; showing the concave deltidial plates and the teeth. X 2. 
Fig. 6. The interior of an incomplete pedicle-valve ; showing the deep pedicle-cavity. X 2. 
Fig. 8. The interior of a brachial valve ; showing the bilobed cardinal process and a portion of one of the 
spirals. X 2. 
Fig. 9. A cardinal view of the same specimen ; showing a narrow area and the size of the cardinal 
pi'ocess. X 24 . 
Fig 10. The cardinal region enlarged; showing the slightly concave deltidial plates, the circular foramen 
nearly enclosed by the substance of the valve, and the lamellae of the surface. X 5. 
Lower Helderberg group. Near Clarksville, N. Y. 
Genus CCELOSPIRA, Hall. 
Page 134. 
CtELOSPIRA PLANOCONVEX A, Hall. 
See Plate .52. 
Figs. 11-13. Ventral, dorsal and profile views of a specimen, natural size ; showing the contour and surface 
characters of the shell. 
Fig. 14. The interior of a brachial valve; showing the form of the cardinal process and the thickened 
median ridge. X 2. 
Fig. 15. A still further enlargement of a portion of the same valve, to show the grooved lobes of the card¬ 
inal process. X 5. 
Fig. 16. The interior of an incomplete brachial valve ; showing cardinal process, dental sockets, and the 
muscular depressions on either side of the thickened median ridge ; enlarged. 
Clinton group. Hamilton, Ontario. 
Genus ATRYPINA, gen. nov. 
Pago 161. 
Atrypina Clintoni, sp. nov. 
Fig. 7. The interior of an incomplete bi-achial valve ; showing the bilobed hinge-plate and the median 
muscular ridge. X 3. 
Fig. 17. A dorsal view of a specimen ; showing the intei'nal cast of the brachial valve and the teeth and 
rostral cavity of the pedicle-valve. X 2. 
Fig. 18. An internal cast of the pedicle-valve ; showing the adductor and diductor muscular scars. X 2. 
Fig. 19. The exterior of a pedicle-valve. X 3. 
Clinton gi'ouji. Drift of western New York. 
Genus CCELOSPIRA, Hall. 
Page 134 
CcELOSPIRA CONCAVA, H;ill. 
Fig. 20. The exterior of a pedicle-valve. X 2. 
Fig. 21. The exterior of a brachial valve ; showing its concavity and the posterior portion of the cardinal 
process. X 2. 
Fig. 22. The interior of the brachial valve ; showing the character of the cardinal process and socket-walls, 
and the median thickening between the muscular impressions. X 3. 
Fig. 23. The interior of an incomplete peiiicle-valve ; showing the open delthyi'iurn, the teeth and muscu- 
lai' impressions. X 3. 
Lower Helderberg- group (Shaly limestone). Near Clarksville, N. Y. 
