433 Costa Rica Miocene— Olsson 261 



valve nearly flat, the right valve much more convex; the right 

 valve carries a strong umbonal keel from the tip of the beaks to 

 the posterior extremity; this keel is but poorl}^ developed on the 

 left valve; anterior end broadly rounded with the dorsal and 

 ventral margins nearly parallel; the posterior side is much nar- 

 rower and with its extremity pointed; the surface is smooth; the 

 dorsal-posterior slope of the left valve is less smooth and carries 

 coarse, irregular growth-lines; the interior is not known. 



Length 54, height 30, semi-diameter 4 mm (a left valve) 

 Length 60, height 27.5 semi -diameter 7 mm (a right 

 valve) 

 This species stands intermediate between the kS*. Smithwood- 

 zvardi Ma ury from the Miocene of Santo Domingo and the re- 

 cent West Indian S. ope^xulata Gmelin. It differs from the Smith - 

 woodzvardi, in being much larger with a less depressed left valve: 

 From operadata the Costa Rican shell has higher and more 

 pointed beaks as well as being longer posteriorly. In full-grown 

 specimens of operailata, the surface generally shows radial lines 

 and striae which may become quite strong near the posterior 

 ventral margin. The Costa Rican shells are smooth except for 

 the growth lines. 



Gatun Stage: Hill No. j, Banana River. 



Genus TAGELUS, Gray 



Tagelus cebus, n. sp. Plate 29, figure 9 



Shell elongate, moderately convex and solid; beaks central 

 with the anterior and posterior ends nearly equal, and with the 

 dorsal and ventral margins parallel and nearly straight; anterior 

 and posterior extremities subtruncate; a broad depressed zone 

 extends from the umbos to the ventral margin of the valves; 

 surface marked with irregular concentric lines of growth; inter- 

 ior concealed. 



Length 36, height 13, diameter of the left valve 3.00 

 mm. 



