196 



Bulletin 39 



368 



This is a medium sized and sickle-shaped oyster with 4 or 5, 

 large, tooth-like folds on the anterior and ventral margins. The 

 beak and ligamental area are directed upwards and sometimes 

 strongly to the posterior side. Its external surface, although 

 strongly and deeply folded, is plain and nearly smooth. 



The Ostrea megodon is a recent species along the west or 

 Pacific coast of Central America. It occurs also as a Miocene 

 fossil in Santo Domingo and in the Bowden beds of Jamaica. 

 Gatun Stage: Water Cay. 



Coll. ^a, East Grape Poiyit Creek. 

 Hill I a, Ba?ia?ia River. 



Ostrea costaricensis, n. sp. Plate 21, figure 2 



Ostrea sculpturata Bose, 1906, Boll, de Inst. Geol. de Mexico, numero 

 22, p. 76, pi. 9, fig. 4. Not of Conrad, 1840. 



Shell of medium size, subcircular in form but somewhat ob- 

 lique; the shell is somewhat flexed or slightly saddle-shaped, the 

 concave side of which corresponds to the left valve; both valves 

 are strongh^ sculptured, the right with narrow, rib-like costse 

 and wider interspaces, the left valve with the reverse of the 

 sculpture of the right; the ribs are more or less nodose or knob- 

 bed and are crossed b}^ irregular concentric lines, which in some 

 cases nia}^ become raised or lamellar-like; the hinge margin is ir- 

 regularly crenulated; 



Height 50, length 47, diameter 19 mm. 



The type is a specimen from the Gatun of the Rio Blanco 

 with both valve tightly closed so that the interior cannot be 

 seen. What appears to be the same species, is represented by 

 several sessile and free valves from East Grape Point Creek, and 

 which are associated with O. megodon. 



It differs from the Ostrea gatuneiisis - haitensis group by its 

 smaller size; less heavy shell and in that the ribs of the valves 

 are narrow and costse-like and not long, strong plicae. Bose 

 has recorded and figured this species as Ostrea sculpturata Con- 



