PALEONTOLOGIC CONTRIBUTIONS 97 



The anterior edge is not preserved; from the extent of the con- 

 centric growth-lines and their sharp curving inward it can be 

 inferred that the anterior margin was, as seen in the compressed 

 condition of the fossil, slightly receding towards the antero-lateral 

 corners, thereby indicating a wide open anterior cleft. 



The growth-lines are very coarse, about four in 5 mm at the 

 posterior extremity and six in the same space in the middle. They 

 are bunched by deeper concentric depressions into irregular groups 

 and distinctly overlap outward. They bend rather abruptly for- 

 ward at the lateral angles, especially in the posterior portion, 

 appearing thus subangular. 



Measurements. Length about 32 mm, width about 38 mm. 



Horizon and locality. Carboniferous (Waverlyan) Cleveland 

 shale, Mill creek, Newburg, Cleveland, Ohio. 



Remarks. The specimen was collected by the late Prof. S. G. 

 Williams and is named in his honor. It is of larger size than the 

 congeners and is readily distinguished by its broad form and coarse 

 lines. The asymmetric shape of the valve is probably due to oblique 

 compression. This is, at least, suggested by the crowding of the 

 growth-lines on the right-hand side and a corresponding exagger- 

 ated spacing of the same on the opposite side of the fossil. 



We have also referred a small valve, reproduced in plate 31, 

 figure 2, to this species because it agrees in its general outline 

 and the character of the growth-lines very well with the holotype 

 of the species. Also in this valve the apex is slightly out of the 

 median line. Along the anterior margin a depressed area is observ- 

 able, in which the surface lines are sharply bent inward exactly as 

 in the holotype. 



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