268 



Bulletin 39 



440 



the keel; the basal margins of both valves are folded in and 

 flattened as seen in C. contrada Say but to a less extent and 

 least in the right valve; surface sculpture of rather heavy con- 

 centric costse, most widely spaced on the umbos; the surface 

 may also show fine radial lines of small pustules or punctse 

 which are best developed on the rostrum; no lunular pit. 



Length 6.5, height 4.25, diameter 4 mm (Banana River j. 

 9 6 semidiameter 2 mm (Saury). 



We have chosen as the type of this species, the smaller and 

 more abundant form as found in the Gatun bed of the Banana 

 River, rather than the larger shells from the more inaccessible 

 Saury. It is related to the C. cojitrada Say, but differs in hav- 

 ing the ventral margin more curved, and the posterior extremi- 

 ty more emarginate. Its surface is sculptured with rather 

 strong, concentric costae which are more widely spaced on the 

 umbos, and submicroscopic radial lines of fine punctae or small 

 pustules, best seen on the rostral areas. 



Gahm Stage: Hills la, j, Banana river (^abundant). 

 Zone G, Saury creek. 

 Coll. 6, Red Cliff' creek. 



Corbula DIetziana C. B. Adams Plate 21, figure 8 



Corbula Dietziana C. B. Adams, Contr. Conch., vol. 12, p. 235. 



Corbula {Cuneocorbula) Dietziana Dall, 1898, Trans. Wagner Free 

 Inst. Sci. , vol, 3, pt. 4, p. 856. 



A recent species quite common in the coralline limestones 

 of the Limon Peninsula, from which it has previously been re- 

 ported by Dall from specimens collected by Hill. 



In its earlier stages of growth, this species resembles the 

 C. ceqziivalvis Phil.^(C cubaniana d'Orb) in its rectangular from 

 and strong dorsal keel. Later in life, the shells become strongly 

 distorted, the ventral margin is abruptly bent downwards nearly 

 at right angles to the former plane of the shell as is seen to a 

 much less degree in^the recent C. contracta Say. Its surface is 



