96 MEMOIRS OF THE CARNEGIE MUSEUM. 



Characters of the shell: Shell rather large and heavy. Outline elongate-ovate, 

 generally about twice as long as high, and often longer. Anterior end rounded, 

 posterior end produced and narrowing, but blunt. Lower margin gently convex, 

 straight, or even slightly concave. Upper margin curving gentty down into the 

 upper posterior margin, without forming a distinct angle, and thus the shell is 

 not subtrapezoidal. Beaks not prominent, situated near the anterior end. Beak- 

 sculpture distinct, consisting of four to five rather heavy bars; the first two are 

 subconcentric, the following ones run in the direction of the growth-lines, are almost 

 straight in the middle, with the anterior and posterior parts obliterated; sometimes 

 an indication of a sinuation is seen a little behind the middle. Shell somewhat 

 swollen in the anterior part and toward the beaks; less swollen and almost flat, 

 sometimes even compressed posteriorly and toward the lower margin, without a 

 distinct posterior ridge. Surface smooth, without any sculpture. 



Epidermis brown to black. In young specimens, traces of greenish rays are 

 discernible, but generally the color is rather uniform, and the growth-rests are 

 likewise not marked by darker color. 



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

 ragged. Interdentum rudimentary or absent. Lateral teeth long. Beak-cavity 

 shallow. Dorsal muscle-scars in the beak-cavity. Nacre white or variably 

 colored from light pink and salmon to deep purple, and coppery purple; often 

 parti-colored, white and salmon, white and purple, salmon and purple, etc. 



No sexual differences in the shell. 



L. H. D. Pr.ct, 



Size: 1. Jamestown, Cat. No. 61.3752 (largest 



at hand) 136 mm. 61 mm. 33 mm. .24 



2. Industry, Cat. No. 61.3763 122 " 55 " 33 " .27 



3. Greenville, Cat. No. 61.3329 91 " 37 " 27 " .30 



4. AVampum, Cat. No. 61.2889 54 " 26 " 13 " .24 



Soft parts (See Ortmann, 1912, p. 271). The Glochidia have been figured by 

 Lea (Obs. XIII, 1874, PI. 21, fig. 10), by Lefevre & Curtis (1910, p. 97, fig. N, & 

 1912, p. 146, fig. 0) by Ortmann (19116, PI. 89, fig. 7) and Surber (1912, PI. 2, 

 fig. 38). 



Breeding season.: I have found several hundreds of gravid females on numerous 

 dates between May 11 and August 13. Glochidia have been seen as early as June 

 5; but on the latest date (August 13) a specimen with eggs was observed. This 

 is clearly an abnormal case. Surber (1912, p. 7) gives the breeding season as from 

 June to August, but it begins before the middle of May. The species is typically 

 tachytictic. 



