PLATE XXI 



PAGE 



Figs. 1, 2. CUCULI^EA VULGAKIS Morton 529 



1. Exterior of left valve of immature specimen. 



2. Interior of same. 



Ripley formation, Ripley, Mississippi, U. S. National Museum. 



Figs. 3, 4. AKCA SAFFORDI Gabb 537 



3. Interior of left valve. X 6. . 



4. Exterior of same. X 6. 



Monmouth formation, Brightseat, Prince George's County. 



Figs. 5, 6. ARCA UANDI Gardner n. sp 539 



5. Cast of double valves from right side. 



6. Anterior view of same. 



Matawan formation, Camp U. & I., Chesapeake and Delaware 

 Canal, Delaware. 



Figs. 7-9. GLYCYMERIS (POSTLIGATA) WOKDKM Gardner n. sub. gen. et sp.. . 543 



7. Interior of left valve. X 5. 



8. Exterior of right valve. X 3. 



9. Posterior view of double valves. X 3. 



Monmouth formation, Friendly, Prince George's County. 



Fig. 10. PTERIA PETKOSA (Conrad) Meek 548 



Cast of left valve. Monmouth formation, Bohemia Mills, Cecil County. 



Fig. 11. PTERIA RHOMBICA Gardner n. sp 549 



Hinge of left valve. X 5. Monmouth formation, 1 mile west of 

 Friendly, Prince George's County. 



Fig. 12. PIXXA LAQUEATA Conrad 545 



Broken cast of double valves. Matawan formation, Post 105, Chesa- 

 peake and Delaware Canal, Delaware. 



917 



