TRAPKZOIDEUS 1 183 



liquely truncate; surface rudely and unevenly concentrically 

 sculptured ; dorsal slope feebly plicate ; epidermis dirty green- 

 ish ; teeth delicate, lamellar ; laterals remote ; nacre bluish, sal- 

 mon-tinted in the cavities. 



Length 60, height 27, diam. 12 mm. 



Length 61, height 28, diam. 16 mm. 



Si am. 

 Unio misellus MorklET, Jl. de Conch., XIII, 1865, p. 21 ; Sen 



Conch., IV, 1875. p. 341, pi. XIV, fig. 2.— von Martens, 



Arch, fiir Naturg, I, 1899, p. 43, pi. vi, figs. 4-8. 

 Trapccoidcns misellus Simpson, Syn., 1900, p. 859. — ^Haas, 



Conch. Cab., Unio, 1912, pi. 32, figs. 6-9. 

 Unio siamensis Lea, Pr. Ac. N. Sci. Phila., X, 1866, p. 133; 



Jl. Ac. N. Sci. Phila., VI, 1868, p. 279, pi. xxxviii, fig. 93 ;. 



Obs., XII, 1869, p. 39, pi. xxxviii, fig. 93. 

 Margaron (Unio) siamensis Lea, Syn., 1870, p. 57. 



I have before me the type of Lea's Unio siamensis, which 

 does not seem to differ in any particular from Unio misellus 

 as described and figured by Morelet, unless it may be a little 

 more compressed. Morelet's measurements, given in the 

 "Series Conchyliologiques," make his species somewhat in- 

 flated and may be incorrect. These are: length 66, height 31, 

 diam. 21 mm. The second measurements I have given are 

 those of his type. 



Var. subchithrattis (von Martens). 



"Differs mainly from the typical form by the folds, which 

 descend from the umbones, but do not reach the basal margin 

 in the centre of the disk; they are nearly vertical and more or 

 less regularly interrupted by the growth-lines and therefore 

 have a granulated appearance, sometimes, particularly near the 

 beaks, they also meet at acute angles pointed downwards, but 

 this is the exception rather than the rule. At both extremities 

 they radiate obliquely, without a noticeable curve and without 

 granulations ; posteriorly they frequently fork and anteriorly 

 often meet at irregular acute angles. The general form and 

 coloring, as a whole, is like typical misellus, but on the average 

 the shell is less thin, somewhat more inflated, the basal margin 



