48 MEMOIRS OF THE CARNEGIE MUSEUM. 



even traces of a breaking up into small zig-zag nodes is seen. In certain other 

 cases the beak-sculpture is separated from that of the disk by a smooth space. 

 Shell sometimes rather flat, but generally more or less swollen, with a broad, dis- 

 tinct, and prominent posterior ridge. In front of the ridge, the shell is flat or 

 depressed, but without forming a distinct radiating furrow. The posterior nodular 

 part of the beak-sculi3ture is continued upon the ridge as large and distinct promi- 

 nent tubercles or nodes, standing rather remote from each other; sometimes 

 these nodes are broken up into clusters of nodules, and in rare cases they are rudi- 

 mentary or absent. In front of the posterior ridge the shell is covered by larger 

 or smaller nodules, very irregular and variable in arrangement, shape, and number. 

 Sometimes there are very few, in other cases very many of them, but toward the 

 lower margin they generally tend to disappear in larger shells, and the anterior 

 part of the disk is always smooth. Towards the beaks, the nodules are smaller, 

 and may disappear, or pass into the beak-sculpture. Posterior slope generally 

 marked off from the posterior ridge by a narrow furrow, producing an emargination 

 of the posterior margin, and its surface may be smooth, or tuberculated, or radiately 

 ribbed. Epidermis yellowish to brownish black, with a characteristic greenish 

 color-pattern: the rays are broken up into triangular spots of larger or smaller 

 size, pointed toward the lower margin. These color-patches are very variable, 

 sometimes almost absent, sometimes very distinct, chiefly so in young specimens; 

 in older ones they become obliterated, the epidermis appearing uniformly brownish. 

 Regular linear rays are sometimes present, but not often. Growth-lines more or 

 less distinct. 



Hinge-teeth well-developed, strong. Pseudocardinals divergent, large, ragged. 

 Interdentum present, generally well-developed. Lateral teeth strong, of medium 

 length. Beak-cavity deep or moderately deep. Dorsal muscle-scars on the 

 hinge-plate. Nacre always white. 



Sexes absolutely indistinguishable in the shell. 



Size; 1. Industry, Cat. No. 61.3866 (largest from L. H. D. 



Pennsylvania) 90 nun. 71 mm. 47 mm. 



2. Kelly, Cat. No. 61.3861 (very flat) 89 " 70 " 36 " 



3. Aladdin, Cat. No. 61.3S68 (normal) 67 " 58 " 38 " 



Soft parts (See Ortmann, 1912, p. 255, fig. 6). Glochidia figured by Lefevre 

 & Curtis (1910, p. 97, fig. E, and 1912, p. 146, fig. E), by Surber (1912, PI. 2, fig. 

 26), and Howard (1914, PI. 5, fig. 31). Their measurements have been given as 

 0.18 X 0.19 mm. and 0.175 X 0.200 mm. I have not seen glochidia in Pennsyl- 

 vania, but have found them in the flat form (var. wardi) from the Little Kanawha 



