THE NAUTILUS. 27 



DESCEIPTION OF A NEW PLEUROBEMA. 



BY L. S. FRIERSON. 



Pleurobema tombigbeanum n. sp. Plate III, figs. 3, 4. 



Shell short, triangular, thick, solid and heavy ; truncated in 

 front, roundly pointed behind at the post-base. Beaks high and in- 

 curved, their sculpture not seen. Post ridge rounded, and close to 

 the post margin. The sides are slightly flattened just in front of 

 the post-ridge, and an inflated, raised area extends from the beaks 

 to the anterior base. This area is to a considerable degree concen- 

 trically sulcated, the sulci becoming obsolete behind, where it be- 

 comes striated; epidermis rayless, dark reddish brown or having 

 faint greenish rays near the beaks, Lunule triangular, and mem- 

 branaceous. The shell is markedly flattened in front, half way from 

 beaks to base, showing a sort of so-called " secondary lunule." 

 Nacre white, to rose color, and iridescent. Muscle scars well im- 

 pressed, and separate. Beak cavities shallow. In the left valve 

 there are two low, thick curved laterals, somewhat striate, and a 

 stout, upright, bifid, striate, acuminate cardinal. In the right valve 

 a single low, stout curved lateral upon a very wide heavy plate, or 

 shelf, and a single wedge-shaped cardinal arising from a pit sur- 

 rounded by a semicircular, low ridge. Cardinal plate thick, on the 

 inner surface of which may be noted the dorsal muscle scars. 



Length 48, alt. 40, diara, 32.3 mm. 



Length 41, alt. 39, diam, 27,5 mm, 



Tombigbee river. Types from Demopolis, Marengo Co., Ala- 

 bama, in coll. Frierson and A. N. S. Phila. Also found at Colum- 

 bus, Mississippi. 



The shell may be mistaken by the casual observer, for a small 

 Quadrula pyramidata, Lea, but may easily be distinguished by its 

 smaller size, and especially its shallower beak cavities, lower beaks, 

 and less pronounced sulcus from beak to post base. It seems a 

 rather rare shell in the Tombigbee and Alabama Rivers. Four 

 and a half specimens were received in three "envois" from the 

 former river, Mr. Bryant Walker informs me that he has two 

 specimens, from the Alabama River. One from the collection of 

 Dr. Lewis, and labeled by him " U. pletius," and the other received 



