197 . 



The ventral is about a fifth longer than wide, judging by these examples, 

 and the dorsal a tenth wider than long. 



This form was observed to have a raised median band on the inside of 

 the dorsal valve which extended in one example four-fifths of the length 

 of the valve. The median band in several examples was traversed by 

 a narrow median groove, made double by a thread-like ridge in the middle ; 

 this groove was interrupted for a short space about one-quarter of the 

 length of the valve from the hinge. Opposite this interruption, on each 

 side of the thickened band or callus, are thinner oval areas of the valve 

 which may be ovarian cavities. 



Mut. EXIGUA, N. mut. PI. XIV., figs. 3 a-d. 



Mutation 



This form is smaller than that last described and shows much varia- exigua from 

 i_. i . , lower horizon 



blllty. described. 



It is about one-third longer than wide ; as preserved in the rock, is 

 rather flat, and the shell substance is comparatively thin. 



The ventral valve is ovate-acuminate, with sides straight for more than 

 half of its length, then slowly and afterwards more rapidly rounded to 

 the front. The usual proportion of length to width is 4 -to 3, but others 

 are more acuminate. Very often the umbo is mutilate, and sometimes 

 the sides of the ventral toward the hinge-line are concave in outline. 

 This valve is flattened toward the front, but more convex toward the 

 beak. Interior. A decorticated example shows a callus extending to the 

 anterior third of the valve, and the points of the lateral vascular ridges 

 are still farther advanced ; the vascular trunks just inside these ridges, 

 curve inward toward the front of the valve. 



The dorsal valve is ovate-cuneate and as long or longer than 

 wide. It is arched down all round, and there is a depressed band along 

 the middle, over the median septum. The interior shows a strong, broad 

 median ridge or callus extending nearly to the front of the valve ; the 

 callus has a longitudinal groove. The lateral septa or vascular ridges are 

 fainter, and between these and the median ridge, just at the cardinal line, 

 are pits of the cardinal muscle. 



Sculpture. This consists of a rough, granulated surface, diversified with 

 ridges concentric to the umbo ; sometimes these ridges are distinctly vi- 

 sible from the umbo out ; on other valves they are seen only from the 

 middle of the valve forward, and are particularly plain and close together 

 near the anterior margin. Oil the dorsal valve toward thn umbo the ends 



