2i8 Bulletin 39 390 



and in number of ribs. The figure of the hinge shows the two 

 cardinal teeth but no laterals. 



The common tridentata Say of the Upper Chesapeake Mio- 

 cene and recent fauna of the West Indies differs mainly in its 

 more triangular form. 



Gatun Stage: Port Limon. 



Genus CHAIViA, (I^inne) Bruguiere 

 Chama congregata Conrad Plate 28, figure 11 



Chavia cong7-egata Conrad, 1833, American Journal of Science, vol. 23, 



p. 341. 

 Chama congregata Conrad, 183S, Fossils of the Medial Tertiary, p. 32, 



pi. 17, fig. 2. 

 Chama congregata Dall, 1903, Trans. Wagner Free Inst. Sci., vol. 3. 



pt. 6, p. 1400. 



Chama congregatoides Maury, 1917, Bull. Amer. Pal., vol. 5, p. 200, 

 pi. 33, fig. 8. 



Our specimens from Costa Rica are all small and belong to 

 the upper or free valve, which in this species is the right. These 

 specimens cannot be distinguished from true congregata of the 

 same size from the eastern United States, where it is an abund- 

 ant fossil throughout most of the Chesapeake Miocene. The 

 upper or right valve is finel}', radiately frilled or fluted and these 

 frills are cut by the concentric lamellae. The attached or left 

 valve is more coarsel}'^ sculptured, both radiall}^ and concentrical- 



ly. 



Gatun Stage: Water Cay. 



Coll. ^, Red Cliff Creek. 



Genus ECHINOCHAMA, Fischer 



Echinochama antiquata Dall Plate 28, figure 8 



Chama antiquata Guppy, 1866, Quart. Journ. Geol. Soc. London, vol. 



22, p. 294. Not of Linne. 

 Echinochama antiquata Dall, 1903, Trans. Wagner Free Inst. Sci., vol. 



3, pt. 6, p. 1404, pi. 54, fig. 9. 



