No. 2.] KILLINGS — PALAEOZOIC FOSSILS, 219 



Ventral valve ovate, the anterior margin broadly rounded, with 

 sometimes a portion in the middle nearly straight ; greatest width 

 at about one-third the length from the front, thence tapering with 

 gently convex or nearly straight sides to the beak, which is acutely 

 rounded. The area is about one-fifth or one-sixth the whole 

 length of the shell, with a comparatively deep groove, which ex- 

 tends to the apex of the beak. The dorsal valve is nearly circu- 

 lar, obscurely angular at the beak, and rather more broadly 

 rounded at the front margin than at the sides. 



In the interior of the ventral valve there are two small muscular 

 impressions of a lunate form, close to the cardinal margin, one on 

 each side of the median line. A second pair consists of two 

 elongate sub-linear scars, which extend from the posterior third of 

 the length of the shell to points situated at about one-fourth the 

 length from the front margin. These scars are nearly straight, 

 parallel or slightly diverging forwards, and divide the shell longi- 

 tudinally into three nearly equal portions. Between them, about 

 the middle of the shell, are two other small obscurely defined 

 impressions. There is also a small pit close to the hinge line 

 and in the median line of the shell. In the interior of the dorsal 

 valve there is an obscure rounded ridge which runs from the 

 beak along the median line almost to the front margin. Close to 

 the hinge line there is a pair of small scars, one on each side of 

 the ridge. The other impressions in this valve have not been 

 made out. 



The surface of both valves is in general nearly smooth, but 

 when well preserved shows some obscure concentric striae. 



This species is closely allied to 0. chromatlca, the species on 

 which the genus was founded, only differing from it, so far as the 

 external characters are concerned, in being much smaller, and the 

 beak of the ventral valve more extended. 



Occurs at Bic and St. Simon. Collected by T. C. Weston. 



0. CiiiGE. — Ovate, front and sides uniformly rounded, posterior 

 extremity more narrowly rounded than the front, length and width 

 about equal, greatest width at the mid-length, rather strongly and 

 uniformly convex, surface nearly smooth, but with fine concentric 

 striae. Length seven lines, width a little less. The rostral por- 

 tion of the shell is much thickened for about one-fifth the leni'th, 

 and in this part there is a deep and wide groove. In front of 

 the thickened portion the muscular impressions are indistinctly 



