384 PROCEEDINGS OF THE NATIONAL MUSEUM. tou5S> 



Doraal and ventral margins nearly parallel. Growth lines numer- 

 ous, fine. Rest periods not sharply defined. Periostracum thin, 

 closely adhering, slightly ruffled along the ventral margin, light 

 chestnut in color, somewhat tawney in the center of the disk. Beaks 

 much eroded. Pseudocardinals two in the right valve, the upper 

 very small, the lower thick, high, and much crenulated. Pseudo- 

 cardinals of left valve two, widely separated, the rear one small, 

 the front one larger and much crenulated. Lateral tooth of right 

 valve high, granulous. Lateral teeth of left valve subequal, crenu- 

 lated. Nacre lurid, purplish-white, thickened in front. 



The type (pi. 51, figs. 3-6) is one of two specimens of United States 

 National Museum, Cat. No. 162425. It measures — length, 45 mm.; 

 height, 27 mm.; diameter, 15 mm.; and comes from Iguape, Brazil, 

 from Dr. H. von Ihering. 



"A small, solid, rhomboid species which almost exactly mimics in 

 external appearance some of the varieties of Unio complanaivs.*'— 

 Simpson. 



DIPLODON PERPLEXUS Simpson. 



Plate 52, fig. 5; plate 53, ligs. 1-i. 



Diplodon perplcxits Simpson, A Descriptive Catalogue of the Naiades or Pearly 

 Fresh-water Mussels, 1914, p. 1248. 



Shell moderately thin, subrhomboidal, regularly rounded in front, 

 obtusely pointed behmd; posterior margin oblique. Ventral and 

 dorsal margins subparaUel. Posterior ridge low, rounded. Beaks 

 but little elevated, located one-quarter of the length of the shell from 

 the anterior margin. Surface of the shell with many fine growth 

 lines. Rest periods feebly marked, except the last which is indi- 

 cated by a dark line and a shallow groove. Periostracum thin, 

 shining, closely adhering, olivaceous, darkening to chestnut anteriorly 

 and posteriorly, slightly ruffled on the posterior dorsal area. An- 

 terior half of shell with numerous, well-marked radiating striae. 

 Nacre milky white and thickened anteriorly, moderately iridescent 

 and thin posteriorly. Pseudocardinals of right valve unequal, the 

 upper one small, the lower one three times as high, crenulated, set 

 at an angle of about fifty degrees with the lateral. Pseudocardinal of 

 rignt valve obhquely flattened and somewhat excavated, its upper 

 margin sharply crenulated. Lateral tooth of right valve thin, high, 

 slightly arcuate, its margin crenulated. Laterals of left valve two, 

 the upper one low, the lower one hijh, the groove between them 

 deep and narrow. 



The type (pi. 53, figs. 1-4) is one of four specimens in the United 

 States National Museum (Cat. No. 1503S9), and measures— length, 

 77 m.; height, 43 mm.; diameter, 26 mm. It comes from Lake 

 Portrero, near Maldonado, Uruguay, and was collected and pre- 

 •ented by Dr. William Rush, U. S. N. Four other specimens and 



