DiPi.onoN 1287 



The only specimen T have seen, the type, is not in first class 

 condition. It is a short elliptic shell almost squarely truncate 

 behind, the truncation meeting the dorsal line almost at a right 

 angle. The epidermis is a peculiar yellow-green with a bronzy 

 hue. It is less elongated than D. solisianus and more trun- 

 cated behind. 



DiPLODON soijsiANUS (d'Orbigny). 



Shell obovate, subinllated, rather thin, wedge-shaped in 

 front when viewed from above, nearly equilateral; beaks full 

 and high, with strong, evenly radial bars, the central ones di- 

 verging; posterior ridge well defined, subangular, ending be- 

 hind in a blunt point on the median line; above this point the 

 shell is obliquely truncated and this truncation meets the dor- 

 sal line with an angle ; anterior end somewhat narrowed and 

 rounded ; surface unevenly but strongly concentrically sculp- 

 tured, with faint traces of radial sculpture, often with strong, 

 radial ridges anteriorly ; posterior slope wrinkled ; epidermis 

 dull brown ; left valve with two compressed, long pseudocar- 

 dinals, which are sometimes united, and two remote, curved 

 laterals ; right valve with two pseudocardinals, the upper small, 

 and one lateral , beak cavities well impressed ; muscle scars 

 small, shallow : nacre bluish-white, dull ; pallial line not im- 

 pressed. 



Length 72, height 53, diam. 34 mm. 



Rio de la Plata and its affluents. 

 Unio solisiana d'ORBiGNV, Guer. Mag., 1835, p. 35 ; Voy. Am. 



Mer., 1843, P- ^^04- pl- LXix, figs. 1-3. — Sowerby. Conch. 



Icon., Xvi, 1868, pi. xciii, fig. 508. 

 Diplodon solisianus Simpson, Syn., 1900, p. 887. 



A rather thin, obovate species with uneven, concentric and 

 faint radial sculpture. The beak sculpture is strictly radial. 



Diplodon variabilis (Maton). 



Shell nearly orbicular but somewhat truncated behind, the 

 truncation joining the nearly straight dorsal line almost at a 

 right angle ; beaks placed about at the center, full but not 

 high, with strong, strictly radial sculpture ; posterior ridge 



