614 



MEMOIRS OF THE CARNEGIE MUSEUM. 



more green toward the beaks, more l^rown toward the margins, but the green 

 may extend over nearly the whole shell, or may be absent. Upon the posterior 

 slope there may be a few indistinct, dark green rays, and the green color near 

 the beaks often has a mottled character, consisting of irregular, darker and lighter 

 concentric bands, and, rarely, of indistinct radial rays or blotches. In some 

 specimens, the anterior and posterior parts of the shell are different in shade, the 

 posterior being darker (more greenish), the two shades being separated rather 

 sharply by a radial line. Growth-rests few, and rarely distinct. 



Hinge-line nearly straight, or very gently curved behind the beaks. In front 

 of them it descends more or less distinctly, and may ascend again at the anterior 

 extremity, thus becoming slightly sinuate. Ligamental sinus triangular, some- 

 what variable, but generally deeper than wide, chiefly in older shells, its anterior 

 margin mostly vertical to the hinge-line, and its lower point curved more or less 

 forward. Often the anterior margin has an S-shaped curve. 



Cavity of shell and beaks moderate. Nacre whitish, iridescent, but none of 

 my sjjecimens has red tints. Radial striations are jiresent, but indistinct in older 

 shells. Prismatic border broad, unequal in width, broadest along the ascending 

 anterior lower margin, narrower in front, and narrowing rather suddenh' at the 

 beginning of the posterior i)art of the lower margin. Its color is grayish or yellowish 

 green, or grayish or yellowish white. Anterior adductor-scar well impressed, 

 subelli])tical, united above with that of the retractor, or sometimes incompletely 

 separated. Anterior protractor-scar generally well separated from the adductor- 

 scar. Posterior adductor-scar less impressed, subovate, with an upper triangular 

 projection formed by the posterior retractor-scar. Pallial line distinct, subcon- 

 centric with the shell-margin, and thus more closely apjiroaching the prismatic 

 border in the anterior jiart of the lower margin than before and liehind it. No 

 dorsal scars. 



Measurements. 



Previous Measurements. 



D'Orbigny (membranacea) . 



Simpson (puUigonira) 



Do. 



Simpson (cra^sa) 



Simpson (vytnani) 



Lea (figure of wymani) . . . . 



