172 ON THE NAIADES. 



Unio parallelopipedon. Plate VIII. fig. 20. 



Testa oblongd, subcylindraced, transversa, valde insequilaterali, postice angu- 

 latd, inflatd, marginibus dorsi et baseos parallelis ; valvuUs subcrassis ; natibus 

 prominidis, retusis ; epidermide fere nigrd ; deiitibus cardinalibus obliquis, cris- 

 tatis ; lateralibus longis rectisque ; margaritd alba et iridescente. 



Shell oblong, subcylindrical, dorsal and basal margins parallel, transverse, very in- 

 equilateral, angular behind, inflated ; valves rather thick ; beaks somewhat elevated, 

 retuse ; epidermis almost black ; cardinal teeth oblique, crested ; lateral teeth long and 

 straight ; nacre pearly white and iridescent. 



Hab. River Parana, Province of Corrientes. 



My Cabinet. 



Cabinet of Dr Burrough. 



Cabinet of the Academy of Natural Sciences of Philadelphia; 



Diam. -9; Length 1-2, Breadth 2-7 inches. 



Shell oblong, subcylindrical, dorsal and basal margins parallel, trans- 

 verse, very inequilateral, flattish on the sides, angular behind, inflated; 

 substance of the shell rather thick ; beaks rather elevated and placed 

 near the anterior margin; ligament long and thin; umbones flattened; 

 umbonial slope carinate ; posterior slope elevated into a carina ; epider- 

 mis finely wrinkled and almost black ; cardinal teeth oblique and crested, 

 larger in the right valve ; lateral teeth long and straight ; anterior cica- 

 trices distinct ; posterior cicatrices confluent ; dorsal cicatrices placed 

 in the centre of the cavity of the beaks ; cavity of the beaks shallow ; 

 nacre pearly white and iridescent. 



Remarks. — This species is from the Burrough collection, and is dis- 

 tinct from any I have seen. It resembles somewhat, in the outline of 

 the margin, the nasutus of Say. The posterior slope does not, how- 

 ever, decline so much, the dorsal and basal margins being nearly 

 parallel. In being subcylindrical it resembles the cylindricus of Say ; 

 it has not, however, either tubercles or arrow-headed markings. The 

 very dark colour of its epidermis is peculiar.* 



* Since the above description and the figure were made, I have seen a more perfect 

 specimen in the possession of Dr Burrough, which has the beaks but little eroded. In 



