Z PROCEEDINGS OF THE U. S. NATIONAL MUSEUM vol. 74 



ridges, those in front of the middle weaker than those in the rear. 

 The middle pairs at each stage of growth fuse into a point, each pair 

 " nesting " into the succeeding pair like V in V. The early shell has 

 numerous concentric threads which are more prominent in the spaces 

 between the ridges than on the summits. On the posterior dorsal 

 area of the young shell there are several obscure flutings running 

 from the hindmost radial ril) to the dorsal margin. 



The type (Cat. No. 368260, U.S.N.M.) measures: Length, 49 mm.; 

 height, 28 mm.; diameter, 16 mm. A paratype (Cat. No. 368261, 

 U.S.N.M) measures: Length, 47 mm.; height, 24.5 mm.; diameter, 

 15 mm. They were presented by Mr. Ralph W. Jackson, of Cam- 

 bridge, Md., and come from Areas, Province of Minas Geraes, 

 Brazil. This locality is on a small tributary, near the source of the 

 Rio Sao Francisco, which flows in a general northeasterly course 

 and enters the Atlantic at about 11° south latitude. Both specimens 

 are unusually well preserved and have the beak sculpture in nearly 

 perfect condition. The species is closely related to Diplodon santa- 

 inouria£ Simpson, but is less quadrate, has the ventral margin more 

 oblique, and has the undulations of the beaks finer, closer, and cover- 

 ing only about half as much area. The color of D. santo.mariae is 

 rich chestnut brown, while D. jacksoni is greenish with metallic 

 luster. Evidently it is related to and groups with D. wagnerianus 

 Simpson (+ ellipticus Wagner), a species occurring in the Rio Sao 

 Francisco. 



DIPLODON PILSBRYI, new species 



Plate 1. iigures 1 aud 3. Plaie 3, figure 2 



Shell rather inflated, oval in outline, rounded in front, obtusely 

 pointed at the rear, rather thin for its size and genus. Dorsal line 

 arcuate, fading into the posterior margin without a pronounced 

 angle, joining the anterior margin at nearly a right angle. Ventral 

 margin curved throughout its whole length, broadly rounding into 

 the anterior margin and making a rounded point with the posterior 

 margin. Beaks at about the anterior quarter of the length. Pos- 

 terior ridge high and rounded and descending gradually to the 

 posterior margin. Descent at the anterior end steep. Sculpture 

 of numerous, concentric lines of growth, the posterior area with two 

 obscure radiating riblets. Rest periods about five, indistinct. Color 

 uniform dark chestnut. Left valve with one pseudocardinal tooth 

 which is long, compressed, and rather low. Right valve with two 

 long, low, compressed pseudocardinals, the groove between them nar- 

 row and shallow. Left valve with two low lateral teeth of nearly equal 

 size, beginning a little remotely from the beak. Right valve with 

 one rather high, thin lateral, its upper margin crenulated. Anterior 

 adductor scars deep, posterior ones well impressed. Beak cavities 



