230 



MEMOIRS OF THE CARNEGIE MUSEUM. 



the posterior slope. There are specimens with well preserved beaks, where there 

 is hardly a trace of these bars. Valves rather convex, chiefly so towards the 

 beaks. No posterior ridge developed. No sculpture upon the disk. 



Epidermis normally greenish olive, lighter or darker, but shading to yellowish, 

 and in old specimens to brownish olive, but never dark brown. Color generally 

 rather uniform, and growth rests not marked by distinct concentric bands. In 

 some specimens, chiefly young ones, faint green rays are discernible, but rays are 

 mostly absent. 



Hinge well-developed. Pseudocardinals heavy and solid, stumpy or slightly 

 elongated, one in right, two in left valve, not distinctly divergent, and becoming, 

 in older shells, subparallel to the laterals. Laterals moderately long, heavy. 

 Interdentum moderately developed. Beak-cavity not very deep. Dorsal muscle*- 

 scars in the beak-cavity. Adductor-scars distinct and deeply impressed, chiefly 

 the anterior ones. Nacre silvery white. 



Sexual differences present in the shell, but not very strongly marked. In 

 the male the lower margin curves up backward from about the middle, and the 

 posterior end of the shell is distinctly narrowed, sometimes almost bluntly pointed. 

 In the female the lower margin begins to ascend at a point back of the middle, 

 and curves up more broadlj^, so that the shell is less narrowed behind, and the 

 posterior end is broader and more evenly rounded. In consequence of this the 

 female shell appears higher and shorter. The males also grow to a size distinctly 

 larger than the females. However, there are females in which the characteristic 

 shape is poorly developed, and sometimes males are more broadly rounded behind, 

 so that it is not always possible to positively tell the sex by the shape of the shell. 



L. 

 Size: 1. Industiy, Cat. No. 61.3561 (gravid 9 ) 66 m 



2. Cooks Ferry, Cat. No. 61.4420 (9 ?) 59 



3. Industry, Cat. No. 61.3561 ( 9 ) 42 



4. Toronto, Cat. No. 61.5444 (cf ) ' 76 



5. Portland, Cat. No. 61.4776 (cf ) 70 



6. 



do. 



do. {(f) 



.60 



Soft parts: Ortmann, 1912, p. 323. Glochidia, ibid., PI. 19, fig. 11; Surber, 

 1912, PI. 2, fig. 25. According to Surber they measure: 0.210 X 0.265; while 

 my figures are: 0.19 X 0.22 mm. 



Breeding season: Gravid females were collected: Aug. 29, 1908 (eggs); Sept. 

 22, 1910 (glochidia); Sept. 24, 1910 (glochidia); and June 20 and 21, 1911 (glo- 

 chidia, discharging). Although these represent only a few dates, it is clearly shown, 



