222 WE^T AMERICAN SHELLS STEARNS. 



to the edge of tbe hiiiule ; of the principal cardinals one is acutely 

 triangular or wedge-shaped and slopes posteriorly, the other is elon- 

 gately triangular, somewhat curved and prolonged anteriorly and below 

 the subsidiary tooth. Between the principal cardinals is a triangular 

 pit into which the central cardinal of the left valve fits ; the left valve 

 also exhibits a slender elongated tooth on each side of the principal, 

 cardinal, which fit into corresponding grooves in the right valve. The 

 muscular scars are large for so small a shell, and the pallial impression 

 is broad and shiny and considerably back from tbe edge of the valves, 

 and the sinus moderately deep and rounded, its upper edge, curving to 

 and touching the lower part of the posterior adductor impression. 



Dimensions : From beaks to ventral margin, 10'"'" ; from anterior to 

 posterior edges, 13""^'. 



Habitat. — Panama Bay, where it was collected by the late Thomas 

 Bridges. 



This little shell, now belonging to the U. S. National Museum (No. 

 102181), was received by the author many years ago with the remains 

 of the Bridges collection of mollusks ; it has not been described hereto- 

 fore for want of access to the literature and material necessary to deter- 

 mine the fact of previous description or the reverse. It is a strongly 

 characterized form, with every aspect of maturity, and quite distinct 

 from any of its nearest allies. 



Order ANOMALODESMACEA. 



Suborder Anatinacea. 



Family Anatinid^. 



Genus PERIPLOMA Scliumacher. 



Periploma discus sp. nov. 



Plate XVI, Figs. 1,2. 



Shell thin, fragile, white, translucent, semiuacreousj inequilateral, 

 nearly circular, being posteriorly subangulated and flexuously squarish 

 and produced ; inequivalve, the left valve being more ventricose thau 

 the right ; valves somewhat gaping ; pallial impression, narrow, shiny, 

 distinct; sinus rather deep and rounded interiorly and curving up to 

 the adductor scar ; beaks small, nearly central, fissured ; hinge a hol- 

 lowed spoon-shaped process (projecting inwards from below the beaks), 

 which holds the cartilage ; this spoon-shaped cartilage cu]) or process is 

 strengthened by an elongated callus slanting anteriorly ; the exterior 

 surface of the valves is finely wrinkled and linearly scabrous (more 

 easily seen by holding a valve up to the light) and otherwise marked 

 by concentric lines and zones of growth. 



