116 MEMOIRS OF THE CARNEGIE MUSEUM. 



Lasmigona (Platynaias) viridis (Rafinesque) (1820). 



Symphynota compressa Lea. Simpson, 1914, p. 481; Lasmigona viridis (Rafin- 

 esque) Frierson, 1915, p. 59. 



Plate IX, figs. 1, 2. 



Records from Peyinsylvania : 



Rhoads, 1899 (Beaver River, Wampum, LawTence Co.). 

 Ortmann, 19096, p. 196, 202. 



Characters of the shell: Moderately large, subsolid, but not verj- heavy. Out- 

 line subrhomboidal, or subtrapezoidal, longer than high. Anterior end rounded, 

 posterior subtruncate. Lower margin slightly convex; upper margin straight, 

 more or less ascending and elevated posteriorly, forming a more or less distinct 

 wing at the upper posterior angle. In young shells the upper jjosterior margin is 

 symphynote. Beaks not prominent, placed in front of the middle of the shell. 

 Beak-sculpture distinct, consisting of about five bars, of which the first or the two 

 first are subconcentric, while those following are distinctly double-looped, with the 

 anterior loop rounded, the posterior angular, and slightly narrower than the an- 

 terior, a sharp re-entering sinus between them. Sometimes the last bars are some- 

 what irregular, interrupted or wavy in their anterior part, but this feature is vari- 

 able and often absent (compare Marshall's figure, 1890, fig. 1). Shell rather flat 

 and compressed, disk only slightly convex. An indistinct, blunt posterior ridge 

 is present. Greatest diameter of the shell at, or immediately in front of, this 

 ridge. Behind the ridge the posterior slope appears more or less compressed and 

 winged. Surface without sculpture. 



Epidermis yellowish to greenish, or, when old, blackish, with more or less 

 distinct green to blackish rays, which are straight, narrow or broad, often con- 

 sisting of bundles of capillary rays. Generally the posterior slope is darker green 

 on account of the more crowded rays. In old shells the raj^s become obscure, and 

 the whole surface turns greenish black. Growth-rests rather distinct, and mostly 

 marked by darker (brown) color. 



Hinge-teeth well-developed, but not heavy, rather delicate. Pseudocardinals 

 lamellate, directed forwards, and almost parallel to the hinge. Interdental pro- 

 jection of left valve well-developed, fitting into an interdental groove of the right 

 valve. Lateral teeth rather long, straight, well-developed, but thin. Beak- 

 cavity shallow. Dorsal muscle-scars in beak-cavity. Nacre white, often cream- 

 color or even pale salmon or flesh-color towards the beak-cavity. 



