8 PROCEEDINGS OF THE NATIONAL MUSEUM vol.69 



upper part; posterior adductor scars well marked but shallow. 

 Pallial line about 5 mm. from the margin. Prismatic border a mere 

 line. 



The type, Cat. No. 85336, U.S.N.M., measures: Length, 25 mm.; 

 height, 19 mm. ; diameter, 121/2 mm. It came from Rio Negro, 1,200 

 miles up the Amazon. It is part of the Isaac Lea collection. 



Dr. I. Lea identified this species as Unio wheatleyanus Lea, and 

 Simpson arranged it in that species in the genus Diplodon. It is 

 true that it bears some resemblance to that species, but careful com- 

 parison with the type of D. wheatleyanus shows that it does not 

 belong there, and the angular junctions of the anterior and pos- 

 terior margins with the dorsal margin, the alate character of the 

 two ends of the hinge line, though slight, and the style of beak 

 sculpture lead to the belief that it belongs in the genus Hyi^. Its 

 habitat in the Amazon further strengthens the belief that it is a 

 Hyria. Lea's type of Diplodon wheatleyanus came from Monte- 

 video, Uruguay. Perhaps the most striking difference between the 

 two is in the beak sculpture, which in Diplodon tvheatleyanus is 

 divergent while in Hyria wheatleyi it is convergent. Judging by 

 its small size and few rest periods the type of this new species is 

 probably immature. It is old enough to show that it differs from 

 all the species hitherto described, especially in its moderately in- 

 flated form, the regularity of the beak sculpture, and the slight alate- 

 ness of the ends of the hinge area. 



DIPLODON ASUNCIONIS, new species 



Plate 2, figs. 2, 8 ; plate 3, fig. 5 



Shell solid, elongately ovate, slightly nasute, somewhat inflated, 

 rounded and narrower in front, truncate and wider behind; dorsal 

 and ventral margins subparallel, lightly curved. Anterior ridge not 

 distinctly differentiated ; posterior ridge high and sharp near the 

 beak, gradually flattening out and becoming nearly obsolete as it 

 approaches the margin. Anterior area full, rapidly descending from 

 the disk of the shell ; posterior area somewhat concave. Anterior 

 margin rounding into the dorsal and ventral margins; posterior 

 margin making an obtuse angle with the dorsal margin, and a blunt 

 point with the ventral margin. Beaks set well forward, somewhat 

 eroded, high, sculptured with 12 very strong radiating ribs which con- 

 tinue to the middle of the shell ; anterior area with three sharp, fine, 

 short radiating riblets; posterior area with five similar but longer 

 riblets. General sculpture consisting of indistinct concentric striae 

 with three rest periods accentuated. There are indications of radiat- 

 ing interrupted riblets. Periostracum dull, closely adhering, and of 

 a uniformly rich chestnut color throughout. Nacre whitish, slightly 



