854 PROCEEDINGS OF THE NATIONAL MI'SEl'M. VOL.XXII. 



tRECTIDENS LINGULATUS Drouet and Chaper. 



*Unio Jingulatus DROUET and CHAFER, Me"ru. Soc. Zool. Fr., V, 1892, p. 148, pi. V, 

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



Borneo. 



tRECTIDENS PROLONGATUS Drouet. 



*Unio prolong atus DROUET, Rev. Biol. Nord Fr., VI, 1894, p. 216; * Jl. de Conch., 

 XLIII, 1895, p. 36.* KOBELT, Abh. Senck. Nat. Ges., XXIV, 1897, p. 87, pi. xi, 

 figs. I, 2. 



Borneo. 



RECTIDENS PERAKENSIS de Morgan. 



* Unio perakensis DE MORGAN, Bull. Soc. Zool. Fr., X, 1885, p. 424, pi. ix, figs. 



3, 4.* PJETEL, Conch. Sara., Ill, 1890, p. 163. 



Perak. 



RECTIDENS PAHANGENSIS Smith. 



* Unio pahangensis E. A. SMITH, Pr. Mai. Soc. Lond., Ill, 1899, p. 315, fig. 



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

 Drouet and Chaper. 



Genus LAMELLIDENS Simpson, 19OO. 

 (Type, Unio marginalia Lamarck.) 



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

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

 radiating line 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 

 bands 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 pseudocardiuals and one lateral; 

 cavity of the beaks rather shallow; dorsal sears 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 Ireite Mahler- Huschel aus Grb'nland, SCHROTER, Flussc., 1779, p. 181, pi. IX, 

 fig. 1. 



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



*? Unio testudinarius SPEXG^ER, Skriv. Selsk. Nat., Ill, 1793, p. 65. * P.ETEL, 

 Conch. Saiu., Ill, 1890, p. 169. 



1 The name groenlandicus can not stand for this species, since Schroter did not use 

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

 India, no Naiad being known from Greenland. 



