﻿LASTKNA 453 



a few coarse, irreg^ular, longitudinal folds; epidermis shining, 

 often rayed ; a single imperfect tooth occurs m each valve, and 

 sometimes vestiges of laterals; there is one roughened dorsal 

 scar in the shallow beak cavity ; muscle scars very large, deep 

 and distinct, the posterior greatly elongated; nacre purplish, 

 shading to blue at the edge ; pallial line radially ridged. 



Animal with very long branchiae, inner and outer about alike 

 in size and form, projecting free slightly behind, the inner 

 free from the abdominal sac nearly the whole length, brownish 

 throughout ; palpi long, large ; mantle thickened at edge, brown, 

 black above ; branchial opening large, with heavy papillae ; anal 

 opening large, without papilke; superanal opening united be- 

 low ; foot very large when living, club-shaped, capable of 

 great extension. No gravid specimens have been seen, but 

 the outer branchiae are probably used as a marsupium. 



Type, Lastcna lata Rafinesque. 

 Lastena lata Rafinesque. 



Shell elongated, subrhomboid, subsolid, compressed, inequi- 

 lateral ; beaks low and compressed but pointed, sculptured with 

 a few strong ridges, which run nearly parallel with the dorsal 

 margin ; posterior ridge low, but often angled, usually ending 

 behind in a point near the base of the shell, but sometimes the 

 hinder part of the shell is much attenuated; above the pos- 

 terior ridge there is generally a radial shallow furrow ; surface 

 with uneven growth lines, sometimes nearly smooth in front 

 of the post-ridge ; epidermis brownish-green, smoky, common- 

 ly having scattered, wide and narrow, broken rays ; hinge often 

 edentulous, sometimes with a feeble, smooth, compressed tooth 

 or ridge ; muscle scars confluent, the posterior ones elongated ; 

 nacre pale blue or purplish, the cavities darker; pallial line 

 showing a small, posterior sinus. 

 Length 88. height 30. diam. 16 mm. 

 Length 75, height 30, diam. 16 mm. 

 Ohio, Cumberland, and Tennessee River systems. 

 Type locality, Kentucky River. 



