RECTIDENS 1159 



not reaching the margin, central parts of the dorsal and ventral 

 margins nearly straight; beaks situated at 1/7 of the length; 

 nacre bluish, pale yellowish in the centre; cardinal teeth com- 

 pressed, elongate, parallel to the margin, slightly rugose, later- 

 als long, straight. 



Length 87, vertical height 31, alae 27, diam. 25 mm/' (von 

 Martens). 



Type locality. Lake Danau Baru, Indragiri, Sumatra. 

 Unio pressirostris von Martens, Nachr. Deutsch. Mai. Ges., 



1900, p. 14. 

 Rectidens pressirostris Haas, Conch. Cab., Unio, 1912, pi. 2y, 



figs. 1-3. 



"Stands in the same relation to U. sumatrensis Dunk, as 

 the European U. platyrhynchus F. Schmidt does to pictoriim." 



Re;ci"idkns prkstoni n. n. 



"Shell closely allied to Unio pressirostris von Martens, of 

 which it may ultimately prove to be a variety, but differing 

 from that species in its less cuneate form and larger size, in its 

 less contracted anterior side and more obtuse and sloping 

 posterior side. 



Long. 31, lat. 85.5, diam. 21 mm." (Preston.) 



Type locality, Perak. 

 Unio perakensis Preston, Rec. Ind. Mus., Vll, 1912, p. 287, 



pi. viii, figs. 5-6. 



"Perakensis" having already been used by de Morgan for a 

 species of Unio, I have changed the name as above. 



Rectidens lingueatus (Drouet and Chaper). 



Shell elongated, scarcely subsolid, subrhomboid, convex or 

 subcompressed, inequilateral ; dorsal and ventral outlines 

 nearly straight and parallel; anterior end angled above, slop- 

 ingly rounded below ; dorsal slope having a long, oblique trun- 

 cation ; posterior ridge not greatly elevated but angled, usually 

 with a smaller ridge above it, ending in a blunt point near the 

 base of the shell ; surface with irregular growth lines, the 

 middle of the disk often granular and occasionally having verti- 

 cal plications ; dorsal slope usually having fine, radial folds ; 



