854 PBOCEEDINGS OF TEE NATIONAL MUSEUM. vol.xxii. 



tRECTIDENS LINGULATUS Drouet and Chaper. 



"* Uiiio lingiilatus Drouet and Chaper, Mdm. Soo. Zool. Fr., V, 1892, p. 148, pi. V, 

 figs. 7-9.—* Drouet, Jl. de Conch, XLI, 1893, p. 42. 



Borneo. 



tRECTIDENS PROLONGATUS Drouet. 



^ Unio prolongatus Drouet, Rev. Biol. NordFr., VI, 1894, p. 216; * Jl. de Conch., 

 XLIII, 1895, p. 36.—*^ KOBELT, Ahh. Senck. Nat. Ges., XXIV, 1897, p. 87, pi. XI, 

 figs. 1, 2. 



Borneo. 



RECTIDENS PERAKENSIS de Morgan. 



*Z7mo perakensis de Morgan, Bull. Soc. Zool. Fr., X, 1885, p. 424, pi. ix, figs. 

 3^ 4._- PiETEL, Conch. Sara., Ill, 1890, p. 163. 



Perak. 



RECTIDENS PAHANGENSIS Smith. 



* Unio imlmngensis E. A.Smith, Pr. Mai. Soc. Loud., Ill, 1899, p. 315, fig. 



Pahang liiver, Malay Peninsula. Exceedingly near to 0. lingulatus 

 Drouet and Chaper. 



Genus LAMELLIDENS Simpson, 1900. 

 (Type, Unio marginaUs Lamarck.) 



Shell elongate, elliptical, pointed behind, with a slight post-dorsal 

 wing, a low, often biangulate posterior ridge, and generally two sharp, 

 radiating lime above it; beaks with curved, radiating ridges which 

 sometimes are slightly zigzag and often become almost concentric, but 

 which fade out as they approach each other at the center of the disk ; 

 epidermis smooth, generally shining, brownish, often with concentric 

 bauds of lighter color, rayless or nearly so ; left valve with two com- 

 pressed pseudocardinals, the front one roughened, and two laterals; 

 right valve with two parallel, lamellar i)seudocardinals and one lateral ; 

 cavity of the beaks rather shallow; dorsal scars few, deep, distinct, 

 scattered; anterior scars separate; nacre bluish white to straw. 



Animal unknown. 



Subgenus LAMELLEDENS Simpson, 1900. 

 (Type, Unio marginalis Lamarck.) 



Shell dark colored ; teeth well developed. 



t LAMELLIDENS MARGINALIS Lamarck. 



*Die hreite- Mahler- Musch el aus Gronland, Schroter, Flussc, 1779, p. 181, pi. ix, 

 fig.l. 



* ? Unio groenlandicus Morch, Am. Jl. Conch., IV, 1868, p. 38.' 



* ? Unio iestndinarius SpemGi^er, Skriv. Selsk. Nat., Ill, 1793, p. 65. — " P.etel, 



Conch. Sam., Ill, 1890, p. 169. 



' The name groenlandicus can not stand for this species, since Schroter did not nse 

 his appellation in anything hnt a descriptive sense. The species is found only in 

 India, no Naiad heing known from Greenland. 



