154 
the position of the heaks varies a little. The umhonial slope is 
often elevated into a rih; and the striae occupy a considerable part 
of the shell and become so profound as almost to denticulate the 
edge of the shell. PI. 4. 
XJnio sulcatus^ Lea. — Specific character. Transversely sub- 
quadrate ; posterior margin not extended beyond the umbones. 
Unio sulcatus, Lea. Trans. Amer. Philos. Soc., vol. 3, N. S. 
Desc. Shell very obliquely transverse^ subquadrate ; with an 
indented groove extending from the umbo to the anterior and 
anterior basal margins, between which the edge is somewhat re- 
tuse ; epidermis yellowish-olivaceous, with radiating, deep green, 
more or less undulated lines ; summit near the posterior extremity, 
prominent ) anterior margin with two obtusely rounded angles, a 
little gaping ; cavity of the hinge membranes behind the summits 
deeply excavated, nearly as long as wide, distinctly angulated be- 
hind ; posterior margin regularly rounded, remarkably short, hardly 
extending beyond the line of the posterior tip of the umbo ; within 
perlaceous-white, more particularly iridescent before ; teeth regular, 
anterior cardinal tooth parallel with the lamelliform tooth, which 
is very slight arquated ; posterior muscular impression deep ; sinus 
of the cartilage very slight, regular ; cavity of the umbo not deep, 
its muscular impressions obvious; palleal impression somewhat 
truncate before, and not extending anteriorly beyond the line of 
the muscular impression. 
Var. a. Within, except on the anterior margin, purple or livid. 
Ohs. It occurs in the Wabash and Ohio rivers. In order to 
avoid confusion, I may mention that the present species is in many 
of the European collections, sent by me within the last five years, 
under the name of fiageUatus. PI. 6. 
Unio hidibundus. — Specific character. Subquadrate ; emar- 
ginate before and denticulated on the anterior basal angle. 
Desc. Shell transverse, subquadrate oval, olivaceous, radiate 
with capillary deep green lines ; summits somewhat prominent, 
nearly terminal : cavity of the hinge membranes oval, rather larger 
behind the beaks than broad : posterior side very short : anterior 
side with a groove, obsolete on the umbo, but becoming deeper to 
the anterior edge, which is consequently emarginate a little below 
its middle; a little gaping; the inferior angle somewhat more 
prominent, rounded and denticulated : anterior muscular impression 
