Palaeontographica Americana 70 



EXPLANATION OF PLATES 



PI/ATE I 



Figure Page 



1. Area hatchetigbeensis Harris 6 



Eocene of Hatchetigbee, Alabama; type specimen; umbo distorted and worn 

 anteriorly; beading of anterior ribs lost in photograph; photographed to show the 

 low, fine ribs extending over the umbonal ridge with two higher, coarser ribs on the 

 middle of the posterior slope. 



2. Interior of the same. 



3. Umbonal view of the same. This well illustrates the form of the beaks and car- 

 dinal area among the True Arks. 



4. Area suhprotracta Heilprin 7 



Type specimen, photographed by G. D. Harris. The lines of growth indicate 

 that in the young the outline is more like that of paratma. 



5. Area paratina Dall 7 



Ohgocene of Bailey's Ferry, Florida; X 3. Many specimens are lower posteriorly, 

 with the greatest height of the shell near the anterior end. 



6. Interior of the same; X 3. 



7. Umbonal view of the same; X 3. 



8. Area oecidentalis Philippi 8 



Recent from Florida; undistorted specimen. 



9. Exterior of the same. 



10. Area oeeidenlalis Philippi; recent from Florida; a more alate, irregular form. 



11. Area oecidentalis Philippi; recent from from the West Indies; umbonal view show- 

 the cardinal area characteristic of the True Arks. 



12. Area umbotiala Lamarck 8 



Oligocene of Bailey's Ferry, Florida. 



13. Area umbonata Lamarck; recent from the West Indies; ventral view. 



14. Right valve of the same. The dark patches between the anterior ribs are due to 

 epidermis. Elsewhere the shading is due to the natural color of the shell. The 

 leafy epidermis remains about the lower part of the umbonal ridge. 



15. Area nmbonata Lamarck; recent from the West Indies; umbonal view. The dark 

 patches at the two ends are epidermis. 



16. Area umbonata Lama.Tck; recent from Sao Paulo, Brazil. The shell is so stained 

 that the radiating lines appear dark. The radiating sculpture is comparatively 

 strong on this specimen and the posterior ribs vary more than is usual. 



17. Interior of the same. 



18. Area Wag7iertana Dall 9 



"Caloosahatchie beds; a specimen with unusually produced wings; Ion. 12.7 

 mm."; after Dall. 



19. "The same, viewed from above"; after Dall. 



