Il6o RECTIDKNS 



epidermis greenish-brown ; teeth dehcate, lamellar, the pseudo- 

 cardinals somewhat strongly vertically striate ; muscle scars 

 shallow; nacre bluish. 



Length 65, height 27.5. diam. 15.5 mm. 



Borneo. 

 Unio lingulatits Droukt and Chaper, Mem. Soc. Zool. Fr., V,, 



1892, p. 148, pi. V, figs. 7-9. 

 Rectidens lingulatns Simpson, Syn., 1900, p. 854. — Haas^, 



Conch. Cab., Unio, 1912, pi. 27, figs. 4-5. 



A rather delicate species, of which a number of examples 

 are before me. In all of them the beaks are deeply eroded, 

 but they are probably not prominent. It is a smaller, less 

 inflated and rougher species than prolongatus. 



Rkctidkns prolongatus (Drouet). 



Shell elongated, subsolid, subinflated, inequilateral; beaks 

 moderately full and high ; dorsal and ventral outlines nearly 

 straight and parallel ; anterior end rounded, angled above ; 

 posterior ridge narrowly double above, each part pinched up 

 into a ridge, fading out behind, ending in a feeble biangulation 

 at and below the median line ; dorsal slope obhquely truncate ; 

 surface with scattered, low, concentric ridges, with faint traces 

 of radical sculpture ; dorsal slope with feeble plications ; epi- 

 dermis dark greenish-brown, subshining ; teeth lamellar, the 

 pseudocardinals much elongated ; muscle scars shallow ; nacre 

 blue, sometimes flesh-color in the cavities. 



Length no, height 42, diam. 30 mm. 



Borneo. 

 Unio prolongatus Drouf.t, Rev. Biol. Nord. Fr., VI, 1894, p. 



2x6.— KoBELT, Abh. Senck. Nat. Ges., XXIV, 1897, p. 87, 



pi. XI, figs. I, 2. 

 Rectidens prolongatus Simpson, Syn. 1900, p. 854. — Haas, 



Conch. Cab., LTnio, 1912, pi. 26, figs. 1-5. 



Much larger than lingnlatus, more inflated, more shining and 

 having more elongated pseudocardinals. The nacre is some- 

 what granularly radiate striate. 



