302 MEMOIRS OF THE CARNEGIE MUSEUM. 



and fifth tend to become obsolete. All the bars are effaced on the posterior 

 slope. Valves more or less regularly convex, more flattened upon the sides. Pos- 

 terior ridge poorly developed, mostly quite obsolete, and indicated only by a 

 stronger convexity of the surface, but sometimes this ridge may become more 

 distinct, chiefly so toward the beaks. Posterior slope gently convex or almost 

 flat, but not concave. 



Epidermis varying from lighter or darker yellowish green, to olive-browoi ; 

 smooth and shining. Rays rarely absent, mostly present, but extremely variable. 

 They are dark green to blackish, straight and continuous, finer or broader, and may 

 cover the whole surface or only part of it. On the posterior slope the epidermis 

 is less smooth, and generally darker. Concentric bands may be present. 



Hinge well developed. Pseudocardinals generally two in each valve, but 

 extremely variable in shape and size. In young specimens they are more or less 

 compressed; in older ones, more stumpy and ragged. Interdentum absent or 

 narrow. Laterals lamellar, high and strong, that of the right valve generally very 

 broad and suddenly truncated posteriorly. Beak-cavity moderate. Dorsal mus- 

 cle-scars in the beak-cavity. Adductor-scars distinct, rather well-impressed, 

 chiefly so the anterior ones. Nacre silvery or bluish white, often with a pink blush, 

 or more or less sufTused with pink or purplish, but never entirely red. 



Sexual differences of the shell wT.ll-marked. In the male, the lower margin 

 is rather regularly curved, and the shell has a blunt posterior point. In the female 

 the lower margin is considerably expanded in the postbasal region, the anterior 

 portion thus becoming almost straight, while the posterior ascends suddenly to 

 the blunt posterior . end of the shell. The whole posterior section of the shell is 

 much higher than in the male, more expanded, and broadly rounded. 



L. H. D. 



Size: (Males) 1. New GalOee, Cat. No. 61.2143 155 mm. 120 mm. 76 mm. 



2. MeadvOle, Cat. No. 61.3392 115 " 74 " 43 " 



3. Cannelton, Cat. No. 61.2881 76 " 52 " 32 " 



(Females) 4. Neville Island, Cat. No. 61.1.582 123 " 88 " 56 " 



5. New Galilee, Cat. No. 61.3236 (gravid) 97 " 71 " 46 " 



6. Darlington, Cat. No. 61.2918 (gravid) 89 " 64 " 40 " 



^oft parts (See Ortmann, 1912, p. 35) figured by Lea (Obs. VII, 1860, PI. 30, 

 fig. 107; Ortmann, 19116, pp. 319, 320, figs. 7, 8). Glochidia (See Lea, Obs. VI, 

 1858, PL 5, fig. 13; Ortmann, 19116, PL 89, fig. 23; Surber, 1912, PL 2, fig. 24). 

 My measurements are: 0.25 X 0.29 mm.; while Surber gives: 0.205 X 0.255 mm. 

 This is a rather unusual discrepancy. 



Breeding season: I have a very complete series of dates for gravid females. 



