224 MEMOIRS OF THE CARNEGIE MUSEUM. 



Characters of the shell: Shell rather small, but thick and solid. Outline irre- 

 gularlj' subcircular, about as high as long, or slightly longer than high, upright. 

 Anterior and lower margins forming a regular curve. Lower margin ascending 

 posteriorly, more or less straight, meeting the obliquely descending posterior 

 margin in a blunt angle in the male. In the female this angle is less distinct, and 

 the posterior margin becomes nearly vertical. Upper margin short, convex. 

 Beaks more or less swollen, slightly elevated, situated close to the middle of the 

 shell, not incurved. Beak-sculpture rudimentary, consisting of about four or 

 five weak bars, slightly sinuated in the middle, angled behind, and disappearing 

 upon the posterior slope. Shell swollen, diameter at least sixty percent of the 

 length. Convexity of valves rather uniform, without any posterior ridge. No 

 sculpture upon the disk. 



Epidermis light to dark brown, in young specimens sometimes yellowish 

 green; the disk is generally uniformly dark brown, while the posterior slope is 

 light brown or yellowish, sharply contrasted with the disk, but the contrast be- 

 comes less distinct toward the lower margin of the shell. In light-colored young 

 shells, there are sometimes faint traces of greenish rays, but generally rays are 

 entirely absent. 



Hinge well-developed. Pseudocardinals strong, stumpy, ragged, triangular, 

 one in right, two in left valve, the latter divergent, and not parallel to the laterals. 

 Interdentum moderate or narrow. Laterals short, strong. Beak-cavity moder- 

 ately deep. Dorsal muscle-scars partly in the beak-cavity, partly upon the hinge- 

 plate. Adductor-scars distinct, and rather deeply impressed. Nacre silvery 

 white, or with more or less pink or light purple inside of the marginal zone. 



Sexual differences in the shell present, but not stronglj^ pronounced, and 

 sometimes obscure. In the male the posterior part of the lower margin ascends 

 and meets the posterior margin in a blimt angle, which, however, may be very 

 indistinct. In the female the lowermost point of the lower margin is situated 

 more backward, and then the margin ascends more suddenly, curving up into the 

 posterior margin more gradually, without forming a distinct angle. Thus the 

 posterior margin appears more vertical, the shell is rather truncate posteriorly, 

 and is rather more elevated. Besides, there is distinctly a difference in size, the 

 female being considerably smaller than the male. 



L. H. D. Pr.ct. 



Size: (Males) 1. Neville Island, Cat. No. 61.1798 59 mm. 52 mm. 38 mm. .64 



2. do. Cat. No. 61.1598 49 " 45 " 32 " .65 



(Females) 3. Charleroi, Cat. No. 61.2840 50 " 48 " 34 " .68 



4. do. " " do 38 " 37 " 24 " .63 



