359 



Costa Rica Miocene — Olsson 



187 



this granulation begins on the sides and on the more anterior 

 ribs, the center is smooth; this granulation is in the form of in- 

 verted V's; their acute apices lying along the center of the ribs; 

 the ribs of the left valve are more or less granulated or beaded 

 throughout and with the ribs on the posterior extremity, double 

 and similiar to those of the right valve; hinge-line straight, 

 with very numerous small, uninterrupted teeth; a rather wide 

 cardinal area wdth 5 or 6 ligamental grooves; ventral margin in- 

 ternally fluted in harmony with the external ribs. 



I^ength 52, height 28, diameter 34 mm. 



52, 32, 17 mm (right valve). 



The form of this shell with its gently curved ventral mar- 

 gin and attenuated posterior extremit}^ is like the Area actino- 

 phora Dall, but is longer and with fewer and heavier ribs. On 

 the other hand, this species may be compared with dariejisis 

 Brown and Pilsbry, from which it differs mojst strikingly by its 

 larger size, wider, higher and more central umbos and more 

 pointed attenuated posterior extremity. 



Gatun Stage: Hill la, Ba?iana River. 



Hill 2, Ba7iana River. 

 Zo7ie 7, Piimbri Creek. 



Area honensis, n. sp. Plate 22, figures 8, 9 



Shell rectangular, elongote, moderately convex; beaks situ- 

 ated at about the anterior 1-4; right valve with about 26, nar- 

 row, smooth or irregularly granulated ribs, separated by inter- 

 spaces of twice their owm width; the left valve with the same 

 number of ribs and similiar to those of the right valve; the ribs 

 of both valves are simple and undivided and if granulated, most 

 heavily on the anterior half of the left valve; interspaces smooth 

 or with irregular, raised concentric threads; cardinal area long 

 and rather wide and with 5 or 6 linear grooves; hinge-lines 

 straight with very numerous, small, uninterrupted teeth; ven- 

 tral margin fluted in harmony with the external ribs. 



