383 Costa Rica Miocene — Oi^sson 



ORDER ANOMALODESMACEA 



Genus THRACIA, Blainville 



Subgenus CYATHODONTA, Conrad 



Thracia Tristani, n. sp. Plate 20, fig. 3 



Shell thin, subquadrate, the left valve slightly convex; the 

 beaks situated at the posterior 1-3 of the shell, are low and in- 

 conspicuous; anterior end, wide and broadly rounded, the poster- 

 ior contracted, depressed and truncated at its extremity; the sur- 

 face is sculptured with regular wave- like folds, which on the 

 middle of the shell are concentric, but are oblique on the anter- 

 ior portion; the}^ are lacking from the posterior area; in addi- 

 tion the whole surface is very finely pustulose and crossed b)' 

 very indistinct growth-lines. 



Length 42, height 30, diameter of the left valve 7.00 mm- 



The Cyathodonta gatunensis Toula of the Canal Zone is a 

 similiar but larger species, reaching a length of 52 mm. It dif- 

 fers from the Costa Rican species in its greater proportional 

 length, the more central beaks and in its sculpture, which ac- 

 cording to Toula' s figure is strictly concentric. 



The C. undulata Conrad, a recent species from the Gulf of 

 California, is sculptured like Tristani with oblique riblets but 

 the left valve is depressed and strongly flattened. 



This species is named for Professor J. Fidel Tristan, Director 

 of the National Museum of Costa Rica. 



Gatun Stage: Rio Betey. 



