244 MEMOIRS OF THE CARNEGIE MUSEUM. 



Upper margin short, convex, continued without any angle into the obliquely 

 descending posterior margin, which meets the lower margin in a distinct, but 

 rounded, lower posterior angle. Beaks moderately elevated above the hinge-line, 

 incurved, located in front of the middle of the shell. Beak-sculpture obscure, 

 consisting of two or three fine and faint, double-looped bars. Valves rather com- 

 pressed, moderately convex anteriorly, much more flattened on the sides, and 

 peculiarly compressed towards the beaks. Posterior ridge very distinct, although 

 rounded, sharpest towards the beaks. Posterior slope, behind the ridge, narrow, 

 truncate, flat, almost concave towards the beaks, but slightly convex towards the 

 posterior end. 



Epidermis greenish yellow to light brown, darker in old individuals, with more 

 or less distinct rays, which are rather narrow, dark brown or dark green, and are 

 generally broken into lunate or squarish, dark spots. In old specimens, the rays 

 may become indistinct, but traces of them and of the spots on them, are in most 

 cases preserved at least in part of the shell. 



Hinge well-developed and heavy. Pseudocardinals two in left, one or two in 

 right valve, heavy, ragged. Interdentum well-developed, rather wide and flat. 

 Laterals heavy. Beak-cavity moderate. Dorsal muscle-scars partly in the beak- 

 cavity, partly upon the hinge-plate. Adductor-scars well-developed and rather 

 deep, chiefly the anterior ones. Nacre whitish. 



Sexual differences of shell not very great, but generally well-marked. The 

 female shell remains considerably smaller than that of the male, is more inflated, 

 so that the posterior ridge is not so sharp, but there is hardly a difference in the 

 outline, except that the posterior end is more rounded. ^^^ 



L. H. D. 



Size: (Males) 1. Cooks Ferry, Cat. No. 61.3565 118 mm. 91 mm. 51 mm. 



2. do. " " do 107 " 83 " 49 " 



3. do. " " 61.4421 94 " 74 " 38 " 



(Females) 4. Cooks Ferry, Cat. No. 61.3565 (gravid) . 84 " 63 " 40 " 



5. do. " " do. (gravid) . 80 " 62 " 38 " 



6. Industry, Cat. No. 61.3568 (gravid) .... 70 " 59 " 37 " 



The above measurements include the maximum records for either sex. 



Soft parts (See Ortmann, 1912, p. 329). Glochidia (Lea, Obs. VI, 1858, PL 5, 

 fig. 8; Lefevre & Curtis, 1910, p. 97, fig. H, and 1912, p. 146, fig. H; Ortmann, 

 19116, PI. 89, fig. 17; Surber, 1912, PI. 2, fig. 14). My (maximum) measurements 



'"Simpson (1900, p. 603) says that the female shell is swollen at the postbasal region. This is 

 hardly correct. The swelling of the female is noticeable chiefly over the lateral faces of the disk, but 

 very rarely in the postbasal region. 



