1224 Diri.ODON 



Prisodon ortonii Lea. 



Shell irregularly obovate or elliptical, compressed, solid, 

 very inequilateral ; beaks apparently rather low and little in- 

 flated ; posterior ridge not greatly elevated, very narrowly 

 rounded, ending in a blunt point just below the median line; 

 dorsal and basal outlines rounded : anterior end with a wing 

 above ; post-dorsal region almost wingless ; surface with mod- 

 erately strong, concentric ridges ; epidermis pale brownish ; 

 pseudocardinals split into a number (in the left valve 8) of 

 denticles, which run in a sinuous direction across the hinge 

 plate ; laterals of left valve two, curved : anterior scars, im- 

 pressed ; nacre bronzy, iridescent. 



Length 135, height 79, diam. 33 mm. 



River Napo, Ecuador. 

 Unio ortonii Lka, Pr. Ac. N. Sci. Phila., XH, 1868, p. i6t ; 



Jl. Ac. N. Sci. Phila., VL 1868, p. 321, pi. 1,11, fig. 134; Obs., 



xn, 1869, p. 81, pi. i.ii, fig. 134. 



Margaron (Unio) ortonii Lea. Syn., 1870, p. 28. 

 Prisodon ortonii Simpson, Syn., 1900, p. 872. 



1. have seen only a single left valve of this, the tvpe. The 

 anterior wing is gone and the epidemiis is somewhat eroded. 

 The shell has been injured when young, which has probably 

 caused a long, low, radial furrow that runs in front of the 

 posterior ridge. The space occupied by the pseudocardinals is 

 not so long proportionally as in typical Prisodon. 



Genus DIPLODON Spix, 1827. 



Diplodon Spix, Test. Fluv. Bras., 1827, p. 33, pi. xxvi. 



Shell elliptical, rounded, elongated or trapezoidal, with rath- 

 er low beaks, wdiich are more or less distinctly radially sculp- 

 tured, the ridges usually curved and approaching below, with 

 a low or scarcely developed posterior ridge ; surface slightly, 

 concentrically sculptured, sometimes broken into fine nodules 

 or corrugations ; epidermis dull, rayless ; hinge with two com- 

 pressed pseudocardinals in the right valve and one slender 

 lateral and two compressed ixseudocardinals, one in front of 



