362 PROCEEDINGS OF THE ACADEMY OF [April, 



stria?. At the basal margin there are several more ribs in the posterior 

 than in the anterior series. Inside there are about 18 teeth in front 

 of the beaks, about 7 behind them. The angle of the hinge line at the 

 beaks is obviously greater than a right angle. 



Length 18, alt. 13, diam. 9.5 mm. 



Described from a right and a left valve, the latter being figured. 

 It is clearly distinct from N. decisa Conrad and N. cordata Dall. of the 

 Pacific slope Miocene, 8 and the recent Pacific N. castrensis Hinds. 



LEDID^3. 

 Leda balboae n. sp. PI. XXVII, fig. 8. 



The shell is rather thin, plump, with small submedian, closely 

 adjacent and recurved beaks; pointed posterior and rounded anterior 

 end, and arcuate base, which is less convex posteriorly. The upper 

 margin is slightly concave behind and convex before the beaks. No 

 lunule. Posterior dorsal area lanceolate, concave, defined by an acute 

 carina running to the rostrum, sculptured with straight radial riblets. 

 The rest of the valve has concentric ribs, which are slightly more 

 abrupt on the upper side. In the middle of the valve in 5 mm. next 

 the basal margin there are about 13 riblets. The hinge-line is rather 

 narrow, armed with angular or V-shaped teeth, of which there are 

 18 behind and 28 in front of the beaks. 



Length 22.3, alt. 12.0, diam. 10.0 mm. (type). 

 " 19.0, " 11.3, " 9.6 " 



Known from two right valves. It has much resemblance to the 

 recent Panamic L. agapea Dall in contour, but differs by the acute 

 ridge bounding the dorsal posterior area and various other details. 



Leda codetta Hinds closely resembles L. balboce, but it differs by 

 having a distinct lunule, which is not present in the Gatun species. 



ARCID^]. 

 Area dariensis n. sp. PI. XXII, fig. 10. 



The shell is long (the alt. six-tenths of the length), basal and upper 

 margins subparallel, beaks small, at the anterior two-sevenths of the 

 length. Valves not quite equal, the left slightly surpassing the other 

 along the basal margin. Sculpture of about 31 radial ribs. In the 

 median part of the valves these ribs are narrower than their intervals. 

 In the left valve the ribs are closely nodulous in the lower part, very 

 shortly scaly near the beaks. Towards the two ends the ribs widen 

 and become divided by a median groove, on both sides of which it 



1 Trans. Wagner Inst., Ill, p. 573, PI. 40. 



