Bulletin 39 370 



We have redescribed this species as Toula had but a speci- 

 men of the right valve. It is a XxxxQ: Peden, distantly related to 

 the Pecten hemicyclicus Ravenel from the Chesapeake Miocene of 

 Florida and the Carolinas, but is smaller and less broad. In both 

 species, the ribs on the right valves of large specimens become 

 divided in the same manner. 



Gatun Stage: Gatun, C. Z. 

 Water Cay, 



Pecten MacDonaldi, n. sp. Plate 16, figures i, 2 



Shell large, subcircular; ears of medium size and equal; left 

 valve is slightly convex due to the middle of the shell being 

 transversely humped or vaulted and a depressed zone follows on 

 the inner slope of each of the raised dorsal submargins; the 

 right valve is slightly but evenly convex; sculpture of the right 

 valve consists of about 26, low ribs which widen out as they 

 approach the ventral margins; their interspaces are at first nearly 

 as wide as the ribs themselves but become only ^ or ^ as wide 

 ventrally; the left valve has 21 or 22 narrow ribs and wider inter- 

 spaces; on the dorsal submargins, the ribs are small and fade 

 away; surface with fine, even, raised lines best seen on the left 

 valve. 



Length 106, height 97, semi-diameter 12 mm. 

 83 80 mm. 



The fragmentary Toro limestone at Gatun and Toro Point 

 contains very few good fossils, although the rock itself is compos- 

 ed almost entirely of broken and ground fragments of shells and 

 barnacles. Dall has described an Epitonium toroensis from the 

 Toro Point limestone where it is fairly common. The Pecten 

 MacDonaldi is found in the Toro limestone which caps the hills 

 just west of the locks at Gatun. 



This species has much the same contour as the recent Pecten 

 maximus Linne, from Europe. It is a larger species than the 

 Pecte7i gatunensis Toula. 



Toro Limestone. Gatun, ivest of the locks. 



