466 AXEL A. OLSSON 



right valve has seven or eight, high and narrow, radial ribs, the posterior 

 ones strongest, widely separated from the next, the size and spacing of the 

 others diminishing forwards; in the left valve the posterior set of larger 

 ribs are double or twinned. 



The doubling of the posterior set of ribs on the left valve is not men- 

 tioned by Sowerby in his description of C. costata. Specimens from Santa 

 Elena, Ecuador, the type locality for this species, show simple ribs on the 

 right valve while they are more or less paired in the left valve. 



Range — Mexico to Ecuador. Panama: Taboga Island. Ecuador: Santa 

 Elena. 



Family POROMTIDAE 

 Generally deep-water species of small size and thin, fragile texture, 

 their shape rounded, cordate, with angular posterior side, pearly within. 

 Ligament with both external and internal elements, the resilium attached 

 to a small chondrophore in the hinge plate. Hinge of the right valve has a 

 short, stout cardinal tooth in front of the chondrophore, the left valve with 

 a smaller tooth behind and above the chondrophore. Surface smooth or 

 granulose. 



Genus POEOMTA Forbes, 1844 

 Type species by monotypy, P. anatinoides Forbes (= Corbula granu- 

 lata Nyst and Westendorp). European Seas. 



Shell with the characters as described above for the family. The species 

 herein considered belong to two subgenera, distinguished as follows: 



I. Surface of shell granulose, the pallial sinus obsolete. 



Subgenus Poromya, s.s. 



II. Surface plain, without granules, the palUal sinus well developed. 



Subgenus Dermatomya 

 The two species of PorOTnya, herein mentioned and discussed, live in 

 relatively deep waters, and as such, are strictly speaking, not members of 

 the near-shore Panamic-Pacific fauna. They have been included in this 

 work in the possibility that these species or some allied forms may come 

 into the hands of collectors from fish trawlers and shrimpers operating in 

 the off-shore waters along the coasts of Panama and Ecuador. 



Subgenus P0R03ITA s.s. 

 Poromya (Poromya) perla Dall Plate 83, figure 10 



Poromya perla Dall, 1908, Bull. Mus. Comp. Zool., vol. 43, No. 6, pp. 428, 429, pi. 18, figs. 

 2, 5 Gulf of Panama in 1270 fathoms, USNM 122,931. 



Shell small, globose, exceedingly thin, whitish, subequivalve, sub- 

 equilateral, with very high, swollen, strongly prosocoelous beaks; no lunule 

 or escutcheon, but the posterior hinge margin of the right valve overlaps 

 that of the left, with a single, strong, radial rib near the edge, which does 

 not appear in the opposite valve; anterior margin of the valves evenly 

 rounded into the nearly semicircular base; posterior slope straight; pos- 

 terior end short, somewhat compressed and attenuated; surface covered 

 with almost microscopically minute granules arranged in radial lines 

 except where removed by friction; the disk of the shell shows their scars on 

 its otherwise polished surface. Lon. 5.7, alt. 5.3, diam. 4.4 mm. 



