92 MEMOIRS OF THE CARNEGIE MUSEUM. 



twice as long as high. Anterior end rounded, lower posterior end more or less 

 produced and bluntly pointed. Lower margin convex, straight, or even sometimes 

 slightly concave in its posterior part. Beaks not greatly swollen and not very 

 prominent, situated in advance of the middle of the shell, but not very near the 

 anterior end. Beak-sculpture very rudimentary, almost obliterated, consisting of 

 a few weak concentric ridges running parallel with the growth-lines. Shell moder- 

 ately swollen towards the beaks, but sides of the disk only gently convex, often 

 quite flat posteriorly. A distinct and rather sharp posterior ridge runs from the 

 beaks toward the lower posterior end. The posterior slope behind this ridge is 

 subtruncate and flat, and toward the beaks it is generally ornamented with fine 

 radiating wrinkles or corrugations. The latter are chiefly seen in young shells, 

 are irregular and often interrupted. Upon old shells they are absent, mostly 

 destroyed by the erosion of the beaks; sometimes they occur also in young speci- 

 mens poorly developed, or are absent. Otherwise the surface of the shell is smooth, 

 without sculpture. 



Epidermis brown, from reddish brown to dark blackish brown; in the young 

 it is lighter brown, and generallj^ marked by dark green, obscure rays, which are 

 narrower or wider, but in adult shells the rays disappear entirely. Growth-rests 

 not distinctly marked by color. 



Hinge-teeth well-developed and very strong. Pseudocardinals divergent, 

 large, and ragged. Interdentum rather narrow. Lateral teeth thick and rather 

 long. Beak-cavity not deep. Dorsal muscle-scars on the hinge-plate. Nacre 

 always some shade of red. Very rarely it is almost white in the centre of the shell, 

 and then some color is always present around the margins. In most cases, the 

 whole nacre is tinted, and the color ranges from a beautiful salmon through all 

 shades of pink and red to a blueish purple. 



No sexual differences in the shell. 



L. 



Size: 1. Kelly, Cat. No. GI.8777 129 mm. 



2. Cooks Ferry, Cat. No. 61.4427 12.5 



3. Kelly, Cat. No. 61.3055 117 



4. Neville Island, Cat. No. 61.926 119 



5. Kelly, Cat. No. 61.3059 89 



6. Shippingport, Cat. No. 61.4607 63 



Soft -parts (See Ortmann, 1912, p. 266, fig. 10). Glochidia (See Ortmann, 

 19116, PI. 89, fig. 6) not quite mature, size: 0.13 X 0.15 mm. Mature glochidia 

 have been described and figured by Surber (1915, p. 8, fig. 13). Their shape is 

 somewhat pointed (subtriangular) ; their size: 0.15 X 0.16 mm. 



