RECTIDENS I161 



Rkctidens perakknsts (de Morgan). 



Shell elongated, solid, inflated, inequilateral, nearly ovate; 

 anterior end slightly angled above ; posterior ridge rather high, 

 subangulate or very narrowly rounded, ending in a point a 

 little" below the median line; beaks only slightly elevated; 

 surface with strong, concentric ridges and very delicate radiat- 

 ing threads ; epidermis thick, brilliant, olivaceous ; pseudocard- 

 inals lamellar, double in the right valve and single in the left ; 

 laterals single in the right valve, double in the left ; nacre 

 whitish-tinted lead-color, somewhat reddish in the beak cavi- 

 ties. 



Length 80, height 30, diam. 22 mm. 



Perak. 

 Unto pcrakcnsis de Morgan, Bull. Soc. Zool. Fr., X, 1885, p. 



424. pi. IX, figs. 3, 4. 

 Rectidcns perakensis Simpson, Syn., 1900, p. 854. — Haas, 



Conch. Cab., Unio, 1912, pi. 29, figs. 2-8. 



More inflated and solid than the preceeding species and 

 having stronger concentric sculpture. 



Rectidens paiiangensis (Smith). 



Shell elongated, convex, subsolid, inequilateral ; beaks very 

 low ; posterior ridge apparently double and ending in a wide 

 biangulation ; anterior end squarely truncate, subangular above ; 

 base line curved ; dorsal slope with a long, oblique truncation ; 

 posterior end almost squarely truncate ; surface concentrically 

 striate ; epidermis fuscous ; teeth lamellar, rather delicate, 

 pseudocardinals roughened ; muscle scars well marked ; nacre 

 pale yellowish-salmon. 



Length 131, height 50, diam. 24 mm. 



Pahang River, Malay Peninsula. 

 Unio paiiangensis E. A. Smith, Pr. Mai. vSoc. Lond., IIL 1899, 



P- 315, fig- 

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



Conch. Cab., LTnio, 1912, pi. 29, fig. i. 



The distinctive characters of this species seem to be the trun- 

 cation of the anterior and posterior ends and the unusually 

 low beaks. In the specimen figured they are slightly eroded, 

 but they do not rise above the outline of the shell. 



