4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 61. 



rounded, descending abruptly to the dorsal area. Escutcheon very 

 narrow, anterior margins flexuous and with a broad lunule unevenly 

 divided, the portion in the right valve being the larger. Dorsal 

 margin arched, ventral margin rounded, angular at the front and 

 back ends. Periostracum closely adhering, unctuous, rather glossy. 

 Color nearly uniformly dark chestnut, very obscurely divided into 

 a lighter anterior and darker posterior portion. Beaks very full and 

 high, eroded, but retaining vestiges of sculpture consisting of stout, 

 radiating ribs. Surface of shell crudely concentrically striated, very 

 obscurely radiated with faint impressed lines, rest periods not ven*^ 

 prominently marked. Nacre silvery white. LoAver pseudocardinal 

 tooth of right valve thin, high, long, and curved, the upper cardinal 

 minute, groove between the two long, broad, and deep. Anterior 

 pseudocardinal of left valve wedge-shaped, thick and large, the 

 posterior one small and low, the groove between the two being small 

 and shallow and narrow. Lateral teeth of both valves much bowed, 

 the right lateral being very stout and high. Laterals of left valve 

 subequal, the groove between them broad and deep. 



The type (Cat. No. 335774, U.S.N.M.) measures: Length C8 

 mm., height 55 mm., diameter 33 mm. It comes from Paysandu, 

 Uruguay, and was presented by Dr. F. Felippone. 



This shell is related to both Diplodon gratus Lea and yatelloides 

 Lea. It differs from both in the squarish form, the truncated front 

 and rear, the very full and high beaks which are nearly cordate, the 

 broad lunule, the pinched and flexuous anterior portion and to a 

 lesser extent in the character of the teeth. The shell has a cordate 

 appearance, which in the young stage must be more marked than in 

 the adult. 



DIPLODON ARCUATUS, new species. 



Plate 1, figs. 4, 6 ; plate 2, figs. 7, 10 ; plate 3, fig. 3. 



Shell subrotund, moderately solid, much inflated, the greatest 

 diameter being located posterior to the beaks, narrowed and shortly 

 rounded anteriorly, very broad and very widely rounded posteriorly, 

 the po.sterior dorsal area broad and sloping and wedge-shaped. 

 Beaks in front of the highest point of the shell, facing forward, 

 eroded but bearing indications of having had several coarse radiat- 

 ing ribs. Periostracum thick, nearlj^ uniformly blackish brown 

 throughout, with very faint indications of radiating striae. Surface 

 of shell roughened by a number of rude, elevated concentric lines 

 of growth. Interior pale pinkish flesh-color, lightly pearly (much 

 pearly luster probably lost). Anterior adductor scars deeply im- 

 pressed, posterior scars superficial. Hinge line greatly arched. 

 Right valve with two pseudocardinal teeth located in front of the 

 beak, the inner one lamelliform, very long and very high, the outer 



