115^"^ RFXTIDKNS 



Small to medium sized. 



Uniform brownish, subcompressed. R. lingulatus. 



Greenish, rayed behind. R. oricntalis. 



Medium sized, convex. R. pressirostris ; prestoni; 



sumatrensis. 

 Large. 



Truncate before and behind. 



R. gracilis ; palembangensis ; pahangensis. 

 Not truncate in front or behind, R. prolongatus. 



Rectidens sumatrensis (Dunker). 



Shell elongate ovate, inequilateral, subsolid, convex, or sub- 

 inflated; beaks rather low, not much inflated; anterior end 

 rounded, a little fuller above; base line nearly straight, full 

 towards the posterior end ; dorsal outline cUrA'^ed to the pos- 

 terior point, which is placed just below the median line; pos- 

 terior ridge rounded ; surface somewhat strongly, concentrically 

 striate, subgranose ; epidermis dark olive-brown or blackish ; 

 hinge line lightly cun^ed ; teeth delicate, compressed, pseudo- 

 cardinals subcrenate ; nacre reddish or violet-tinted, beautifully 

 iridescent. 



Length 74, height 30, diam. 20 mm. 



Lake Dana Luar, Sumatra. 

 Unio sumatrensis Dunker, Zeits. fiir Mai., IX, 1852, p. 52. — 



Peeiffer, Nov. Conch., H, 1866, p. 152, pi. xxxix, figs. 



10-12. 

 Rectidens sumatrensis Simpson, Syn., 1900, p. 853. — Haas, 



Conch., Cab. Unio, 1912, pi. 28, figs. 3-6. 



This species, which is quite evenly long ovate, has delicate 

 granulations on a part of its surface, according to Dunker. 

 I have seen a shell, which is probably this species, which is 

 richer-colored within and without, and is more nearly ovate in 

 outline than the other members of the group. 



Rectidens pressirostris (von Martens). 



"Shell elongate, concentrically striate and distantly lirate, 

 dirty brown, anterior end inflated, rounded, posterior com- 

 pressed, obtusely rostrate, posterior ridge well marked, but 



