PLATE LVII 



PAGE 



Fig. 1. LEPT^EXA RHOMBOIDALIS VAR. VEXTRICOSA (Hall) 309 



Interior of ventral valve. Oriskany formation, Ridgely member, 

 Cumberland. (After Hall.) 



Figs. 2-6. LEPT^NISCA CONCAVA (Hall) 310 



2. Ventral exterior. 



3. Dorsal aspect. 



4. Enlargement of the surface, showing the finer longitudinal striae. 



New Scotland formation, New York. 



5. Exterior of ventral valve. X 2. 



Helderberg formation, New Scotland member, Devil's Back- 

 bone. 



Fig. 6. STROPHEODONTA COEYMANENSIS Swartz n. sp 313 



Ventral valve showing exterior save in center where muscular scars 

 of interior are seen. Helderberg formation, Coeymans member, 

 Devil's Backbone. 



Figs. 7-10. STROPHEODOXTA ARATA (Hall) 311 



7. Side view. 



8. Ventral valve more or less exfoliated. 



9. The surface showing ridges and striae. 



New Scotland formation, New York. 



10. Small ventral valve. Helderberg formation, Coeymans member, 

 Devil's Backbone. 



Fig. 11. STROPHEODOXTA DEMISSA (Conrad) 313 



Internal cast of ventral valve. Oriskany formation, Ridgely member, 

 Pendleton County, W. Va. 



Figs. 12, 13. STROPHEODOXTA (LEPTOSTROPHIA) BECKII (Hall) 314 



12. Ventral exterior. 



13. Ventral interior. 



New Scotland formation, New York. 



Figs. 14-16. STROPHEODOXTA (LEPTOSTROPHIA) PLAXULATA (Hall) 315 



14. An individual showing faint concentric undulations. 



15. Ventral valve partially exfoliated. 



Coeymans formation, New York. 



16. Ventral valve differing from usual form. 



Helderberg formation, Becraft member, Licking Creek, 

 Penna. 



Figs. 17, 18. STROPHEODOXTA (LEPTOSTROPHIA) BIPARTITA (Hall) 316 



17. Exterior of ventral valve. Pinto. 



18. Interior of ventral valve. Devil's Backbone. 



Helderberg formation, Keyser member. 



