ART. IG. NEW FEESH WATER MUSSELS ^.lAESHALL. b 



one very small, the pit between the two teeth at a considerable dis- 

 tance from the beak. Left valve with a long, thin, tonguelike 

 pseudocardinal. Right valve with a very short, high, curved lateral 

 tooth. Left valve with two lateral teeth, the groove between them 

 long and shallow, the inner tooth thin and high, the outer tooth 

 thin, low, and obscure. Upper edges of the teeth minutely crenu- 

 lated. Prismatic border very narrow. 



The type (Cat. No. 341376^ U.S.N.M.) measures: Length 39 mm., 

 height 34 mm., diameter 23 mm. It comes from Barra de Sacra, De- 

 partment of Paysandu, T^uguay, and was collected and presented 

 by Dr. Florentino Felippone. 



Externall}- this shell in a general way resembles l>rplodon discidus 

 Lea, except that it is much more inflated and has no indication of a 

 posterior wnng. The external features which attract attention are 

 its inflation, the flattened, sloping posterior dorsal area, the apparent 

 tilting forward of the umbonal area and the uniform dark color of 

 the periostracum. Internally the shell widely differs from any 

 hitherto known Diplodon. In the character of the hinge line and 

 teeth it does not fall into any of the groups into which the genus 

 has been arranged and for the present it must be the sole occupant of 

 a new group. 



DIPLODON TRIVIALOIDES, ne^v species. 



Plate 1, figs. 12, 14 ; plate 2, figs. 3, 5 ; plate 3, fig. 12. 



Shell subelliptical in outline, moderately inflated behind the 

 middle, rather thin posteriorly, somewhat thickened anteriorly, 

 rounded and narrower and slightly gaping in front, oblicjuely trun- 

 cated and obscurely sinuated behind. Dorsal margin slightly arched; 

 ventral margin oblique, nearly straight; body of shell broadly but 

 slightly compressed from the beaks to the middle of the ventral 

 margin; posterior ridge wide, inflated, evenly rounded; posterior 

 dorsal slope with two obscure, broad and shallow radial grooves. 

 Color brownish olive and greenish, with very obscure lighter rays; 

 a fairly well-marked light ray from the beaks to a point just be- 

 hind the middle of the ventral margin divides the coloring into two 

 parts, the anterior portion being the lighter in color. Beaks full 

 and high, well forward, concentrically striated, and with a niunber 

 of heavy radiating ribs, those on the anterior and posterior areas 

 being somewhat threadlike. Periostracum closely adherent, smooth, 

 unctuous, with a dull gloss. Cavity of shell moderately deep. Pos- 

 terior cardinal tooth of right valve high, its surface crenulated. 

 Anterior right cardinal thin, long, inconspicuous. Cardinal of left 

 valve narrow, its lower end buttressed to the upper edge of the ad- 

 ductor scar. Right lateral high, thin, winglike, crenulated. Left 

 laterals close together, outer one small, inner one rising to promi- 



