PRIMORDIAL PERIOD. 35 



sides regularly, and front broadly, rounded; posterior margin slightly 

 straightened, forming a comparatively short, slightly convex, or nearly 

 straight hinge-line. 



Dorsal valve flattened ; beak marginal or nearly so, not prorhinent ; 

 interior surface having a slightly-elevated median -ridge, beginning beneath 

 the beak and extending to about the middle of the valve, where it disap- 

 pears. 



The condition of all the specimens of this species which the collections 

 contain is such that the muscular impressions are not distinctly shown, but 

 those of the posterior adductors appear to be small, and placed nearly 

 beneath the beak, one on each side of the median ridge just mentioned ; 

 between these muscular impressions and the posterior margin there is, at 

 each side, an obscure diverging ridge, or fold, which seems to blend with the 

 postero-lateral margin. 



Ventral valve moderately convex in the umbonal region, but more 

 flattened anteriorly and laterally; beak excentiic, somewhat prominent, 

 and minutely perforate. Some of the specimens show what appear to be 

 small addiictor impressions placed in the apex, close to the foramen, one 

 at each side of it. One specimen shows a slight flattening of the space 

 upon the outer surface, between the apex and the hinge, producing the 

 appearance there of an indistinctly-defined cardinal area. 



The inner surface of both valves of all the specimens of this species 

 contained in the collections has been more or less exfoliated by weather- 

 ing, whereby some of the principal characters have been obscured. Con- 

 sequently, the foregoing description is not only incomplete, but it is probable 

 that the discovery of more perfect specimens may show necessity for modi- 

 fying it. The cast of a single valve found associated with those used in 

 this description, showing large and distinct muscular impressions, akeady 

 suggests such a modification; but its characters are not embodied in the 

 description, because that specimen is not certainly known to belong to the 

 species. The specimen referred to is illustrated by figure 3 d., Plate I. 

 The other specimens all show fine radiating .lines in the structure of the 

 shell, and also concentric lamina? of growth. They are all compressed in 

 dark shale, and show only the interior siirfaces of the valves, none showing 



