104 MEMOIRS OF THE CARNEGIE MUSEUM. 



direction of the growth-lines, being subparallel and nearly straight in the middle, 

 and curving up in front and behind, the posterior curve being angular, with a 

 slight swelling (See Marshall, 1890, fig. 6). Shell not, or very little, swollen, gen- 

 erally more or less compressed, chiefly so on the posterior slope, with a blunt, more 

 or less distinct posterior ridge. Sides of the disk generally flat. Surface without 

 sculpture. 



Epidermis yellowish, to brown and blackish. In young specimens, more or 

 less distinct greenish rays are present. The epidermis rarely is lighter and greenish 

 in color, with more distinct rays. The latter are capillary or somewhat broader, 

 straight. In older shells, all traces of rays generally disappear. Growth-rests 

 more or less strongly marked, but often quite indistinct. 



Hinge-teeth well-developed, but not very heavy. Pseudocardinals divergent, 

 ragged. Interdentum practically absent. Lateral teeth long, gently curved. 

 Beak-cavity very shallow. Dorsal muscle-scars in the beak-cavity. Nacre white 

 or colored, with an immense range of variation, through all shades of salmon, pink, 

 red, purple, and blueish, with coppery or bronze lustre or iridescence; often whitish 

 with lurid tints (greenish and grayish, as if discolored). 



No sexual differences in the shell. 



L. H. D. 



Size: 1. Flinton, Cat. No. 61.3797 (largest from 



Pennsylvania) 120 mm. 65 mm. .32 mm. 



2. Tioga, Cat. No. 61.4312 110 " 57 " 25 " 



3. Selinsgrove, Cat. No. 61.4643 100 " 58 " 35 " 



4. Manayunk, Cat. No. 61.1832 88 " 47 " 27 " 



Soft parts (See Ortmann, 1912, p. 269). Glochidia: ibid., PI. 19, fig. 1. 



Breeding season: For gravid females the following dates are at hand: April 

 26, 1909; May 3, 1909; May 4, 1909; May 6, 1912; May 9, 1911; May 10, 1912; 

 May 11, 1912; June 3, 1912; June 4, 1912; June 5, 1912; June 7, 1912; June 10, 

 1912; June 12, 1912; June 13, 1912; June 14, 1910; July 9, 1914; July 11, 1914; 

 July 16, 1908. Glochidia have been found as early as June 7, and discharging 

 females have been observed on June 7 and 13, and July 9 and 11. According to 

 these dates this is a tachytictic form, breeding from the end of April to July, but 

 the season may extend to August, as Conner (1907, p. 88) states. The discharging 

 females expelled their placentae whole into the surrounding water. 



Remarks: This species is quite characteristic and easily recognized, in spite 

 of its immense range of variation. The trapezoidal outline (with the upper and 

 posterior margins forming an angle), and the flat sides of the shell are reliable 

 features, and in most cases the shell is distinctly compressed. If more swollen, 



& 



