PLATE XX 



PAGE 



Figs. 1, 2. PERISSOXOTA LITTLII Gardner n. sp 523 



1. Double valves from left side. X 2. 



2. Dorsal view of same. X 2. 



Monmouth formation, Brightseat, Prince George's County. 



Figs. 3, 4. NEMODON EUFALEXSIS (Gabb) Conrad 525 



3. Interior of left valve. X 3. 



4. Exterior of same. X 3. 



Monmouth formation, Friendly, Prince George's County. 



Figs. 5-7. NEMOIJON CECILICS Gardner n. sp 528 



5. Cast of a left valve. X iy a . 



6. Cast of left valve. X 2. 



7. Squeeze taken from natural mold of sculpture. X 2. 



Monmouth formation, Fredericktown, Cecil County. 



Figs. 8, 9. CUCULL^:A VULGABIS Morton 529 



8. Interior of left valve of adult. 



9. Exterior of same. 



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



9JG 



