Description of a new Unio from Borneo

Shell somewhat squarely oblong, moderately inflated, very inaequilateral, covered with a dark-brown fibrous epidermis, giving to the shell the appearance of being concentrically striated; striae coarser towards the margins, if seen by transparent light the colour is yellowish-brown. The umbones being decorticated, no sculpture is perceptible on these parts, but the posterior slope is provided with radiating folds, usually becoming bifurcate towards the margin. Upper margin moderately curved, anterior margin rounded, passing insensibly into the rather straight ventral margin; posterior margin sinuous above, then abruptly truncate, forming two angles, one at the upper part of the truncation and one at the beginning of the ventral margin; from this point an obtuse ridge runs towards the umbones. Nacre iridescent, bluish white near the margins, olive yellow near the umbones. Hinge strong, one thick tooth in the right valve, with crenated edge, two in the left valve, the posterior one being the largest; both are obsoletely crenated. One lamella in the right valve, two in the left one, they are slightly curved, somewhat crenated posteriorly. Anterior scars irregular, deep, posterior ones shallow, pallial line distinct. A flat callosity runs from the umbones, obliquely towards the ventral margin. Long. 69, alt. near the umbones 37, at the angle of superior and posterior margin 40, diam. 19 mill.