363 Costa Rica Miocene— Olsson 191 



submargins strongly depressed or flattened with a straight post- 

 erior margin; the left valve with 30 smooth and strongly nodul- 

 ous radial ribs of which 9 are situated on the posterior sub- 

 margin; the ribs of the posterior submargin are low, wide and 

 nearly smooth; on the rest of the shell, the ribs are nodulose, 

 very regularly in the middle of the valves more distantly on the 

 anterior ribs; the interspaces between the ribs are narrow on the 

 middle of the valves, become wider anteriorly and on the ex- 

 treme anterior extremity equal the ribs in width; the cardinal 

 area is high, as nearly }^ of the length. 



lycngth 39, height 33, diameter of the left valve 15 mm. 



The above description is based on a single left valve from 

 the Upper Gatun beds near Cahuita. Distantly related to the 

 Area incongriia Say and its southern form brasiliana Lamarck, 

 the fossil species differs in its higher and more convex shell, and 

 in its high cardinal area. In this latter feature, this species is 

 like the recent West Coast Area labiata Sowerby, but has a 

 greater number of ribs and different form, 



Gatun Stage: Aeross the divide from Comadre Creek. 



Groups of Area Pittieri Dall 



This is a group of small and medium-sized Arks, containing 

 3 species, whose general appearance is that of the recent Area 

 Chemnitzi Phil. The Costa Rican shells are usually abundant 

 wherever they occur in the Gatun formation. Their relations 

 and differences ma}^ be summarized as follows: 



A. Both valves more or less similiarl)^ sculptured, that is 

 the ribs of the posterior half of the right valve, at least obsolete- 

 ly beaded. 



B. Ribs 25 to 28; umbos high and full; posterior-dor- 

 sal slope, angular; posterior extremity somewhat produced, 

 giving an elongate outline to the shell. 



Area Pittieri Dall 



A A. Valves not similiarly sculptured; posterior half of 



