II 86 TRAPKZOIDEUS 



I am not certain as to the systematic position of tliis species, 

 which may belong here. It is a rather small, oblong, sub- 

 rhomboid form with concentric sculpture, apparently some- 

 what corrugated and having a yellowish epidermis. 



Trapkzoidkus rivNiNSLn.AHis Simpson. 



Shell subrhomboid, rather thin, inequilateral; beaks but 

 slightly elevated ; posterior ridge double, ending in a faint 

 biangulation at and below the median line; dorsal slope 

 obliquely truncate ; anterior end narrowed and rounded, cut 

 away a little below ; base line curved ; surface finely wrinkled 

 and apparently concentrically sculptured ; epidermis fulvous- 

 yellow or fulvous-olive. 



Length 39, height 23 mm. 



Sumatra. 

 Unio s}(inati'cnsis Sowkrby, Conch. Icon., X\''I, 1865, pi. 



XXVIII, fig. 142. 

 Trapesoidciis pcninsularis Simpson, Syn., 1900, p. 859. 



I am not at all certain where this species should be placed, as 

 Sowerby's description is very inadequate. It is shorter in pro- 

 portion to height than the other species, and the base line is 

 more rounded. This is not Lea's sumatrensis, nor that of 

 Dunker. The name being preoccupied, I have changed it to 

 pcninsularis. I do not think it is the same as the Unio couiptus, 

 as Fischer believes. 



Trapezoideus ludovicianus (Rochebrune). 



■'Shell ovate elliptical, thick, moderately convex, brown; 

 anterior region rounded ; dorsal margin arcuate behind, ab- 

 ruptly truncated below ; ventral margin subsinuous ; surface 

 deeply, concentrically sulcate, with minute, straight, interrupted 

 grooves, which toward the dorsal margin and especially at the 

 umbonal region develop into conspicuous, intricate waves; 

 umbones obtuse, greatly eroded, coppery ; cardinal tooth 

 quadrate, thick, crenulate ; lateral teeth elongate, distant in the 

 right valve ; interior nacreous, bluish, salmon under the beaks. 



Length 50, height 32, diam. 22 mm." (Rochebrune). 



Type locality, Preck-Scholl, Ll'pper Mekong. 



