FOSSILS OF THE GOAL MEASURES. 507 



slightly and evenly concave; binge line a little less than the 

 greatest transverse diameter; cardinal process rather stout, 

 with an obscure linear ridge (or sulcus) extending forward 

 from its base nearly to the front ; cardinal edge slightly 

 thickened within, so as to form a faintly denned ridge ex- 

 tending about half way from the cardinal process toward 

 each lateral margin, but apparently without any trace of 

 sockets for the reception of teeth in the other valve; mus- 

 cular and other internal markings unknown; surface orna- 

 mented by numerous slender, exceedingly regular, closely 

 arranged concentric lines, exactly parallel with each other 

 and the front and lateral margins. Ventral valve unknown. 

 Length of a medium sized specimen, 1.30 inches; breadth, 

 2.i>5 inches. Largest examples seen, 2.10 inches in length, 

 and of nearly the same proportional breadth as the others. 



Of this very remarkable shell, we have seen six or eight specimens, and 

 some fragments of others. All of the specimens yet fouud, however, 

 are dorsal valves only, the ventral valve being entirely unknown to us. 

 The slightly worn, or more or less weathered condition of the specimens 

 has obliterated whatever muscular or other internal markings there 

 may have been. In most cases only patches of the shell itself remain, 

 though even in these cases the general outline and external surface 

 markings are very distinctly defined in the matrix. All the specimens 

 show a rather obscure, linear, internal sulcus, extending from the base 

 of the cardinal process nearly to the anterior margins. This, however, 

 is probably caused by the accidental removal of a linear mesial ridge, 

 because we also see it equally defined in impressions of the external 

 surface of the valve left in the matrix, just as would be the case if a 

 firm internal ridge had been, owing to the thickness of the shell, as it 

 were, pressed through. The concentric lines of the surface present an 

 extraordinary degree of regularity, both in size and arrangement. On 

 a medium si/ed dorsal valve about sixty of these lines may be counted, 

 while some of the largest show twice as many. They are of exactly the 

 same sixe and distance apart on all the specimens, and of so nearly the 

 same size on all parts of the same individual, as to appear to the eye to 

 be exactly uniform throughout. By measurement, however, we count 

 fifteen of them in the space of 0.25 inch, near the margins of a medium 

 si /.cd speeimen. and twenty in the same space near the beak. In a few 

 instances we have observed what seemed to be the faintest possible 

 traces of a few larger radiating lines of cost a- near the middle of the 



