33 
the anterior muscular impressions are confluent, and 
the posterior impressions distinct. 
Not uncommon in the head waters of the Black 
Warrior river. 
I have given this species the name of my friend, 
Jacob Green, M. D., Professor of Chemistry in 
Jefferson College, a gentleman well known as a 
contributor to the science of Conchology. 
UNIO PEN IT US. 
Plate Y. — Fig. L 
DESCRIPTION. 
Shell triangular, very thick, except on the posterior 
side, ventricose; umbones flattened; beaks prominent 
and rounded; umbonial slope angulated; posterior 
margin flattened; a broad convex elevation com- 
mencing near the base and projecting on the poste- 
rior extremity, with radiating prominent lines, which 
dentate the margin, and concentric grooves; epider- 
mis olivaceous, wrinkled; ligament very short; car- 
dinal teeth double in both valves, thick, prominent, 
sulcated and granulated; posterior muscular impres- 
sion very large and deep; nacre very white; irides- 
cent in the cavity of the umbonial elevation; cavity 
capacious. 
OBSERVATIONS. 
This shell has much the form of U. triquetev of 
