PLATE 12. 



PilgO 



Fijr. 1 - (>. ()KTItI8 I'LANOCONVEXA. lliS 



1 «, b. Views of small individuals of this species, from Maryland. 



2 a, b, c, d, & 3. Largor individuals, from Mew-York, showing the ventral and dorsal sides. 



l>roflle and area. 



4. The interior of the dorsal valve of a smaller individual. 



5. Interior of the ventral valve. 



6 a, b. Dorsal and ventral valves of a cast of the same species. 



Fig. 7 - 21. Okthis subcarinata. 1(19 



7, 8 a, b, c. Views of young individuals. 



0, 10. Views of individuals which have a subquadraugular outlmc, with the dorsal valve 

 deeply depressed in the centre and much elevated on each side, and the front margin 

 strongly sinuate. 



11. 12 Interior of tlie valves of the preceding specimens. 



13. 14, 15. Individuals showing gradation in size, and some slight modification in proportions. 



10, 17. Full-grown individuals of this species. 



18. Interior of the ventral valve. 



19 a, b. Interior of the dorsal valve, showing the muscular and vascular impressions, and car- 

 dinal view of the same. 



liO. 21 a, b. c. Ventral, dorsal, and profile views of casts, the vascular impressions, etc. 



