256 Bulletin 39 428 



end longer with a sloping dorsal margin and rounded anterior 

 extremity; posterior side shorter, with a dorsal margin sloping 

 more rapiding to the wide and flexed posterior end; the left valve 

 is convex but depressed along the narrow posterior submargin or 

 wing; the right valve is less convex with a broad depressed area 

 extending from the beaks to the ventral margin; this depressed 

 zone is sharply separated from the posterior-dorsal slope or wing 

 by a strong angle or flexure which corresponds to the sinus in 

 the middle of the wing on the left valve; surface finely striated 

 with concentric growth-lines. 



IvCngth 48, height 37, diameter 19 mm. 



Pilsbry and Johnson have described without figuring, 

 two species of Metis from the Miocene of Santo Domingo (the 

 M. efferta and postrevia) . These species, they compare with the 

 M. chipolaiia Dall, from the Chipola beds of Florida and men- 

 tion in their description the nearly central location of the beaks. 

 In the Costa Rican shell the beaks are at the posterior one-third 

 of the shell. It is very distinct from the M. chipolana Dall. M. 

 trinitaria Dall from Trinidad and Porto Rico differs in its more 

 central beaks, fuller umbos and has the right valve much less 

 flexed posteriorly. 



Gatun Stage: Betey creek. 



Genus iVlACOMA, Leach 

 Riacoma costaricana, n. sp. Plate 26, figures 14, 16 



Shell broadly ovate, thin and slightly inflated; dorsal and 

 ventral margin straight and parallel; the posterior dorsal side 

 sloping to the rounded ventral extremity; anterior extremity 

 rounded; the posterior side is very slightly flexed; surface 

 smooth except for very fine, incremental growth lines; interior 

 of shell concealed. 



Length 29, height 19, semi-diameter 4.5 mm. 



The reference of this species is Macoma is doubtful as the 



