UNIO 5O7 
shell, which run nearly parallel with the laterals; muscle scars 
very deep and distinct. 
Marsupium occupying the entire outer gills; branchie large, 
curved below, inner the wider except at the posterior end, free 
from the abdominal sac only part of their length; mantle very 
thin, with thickened edge; branchial opening generally large. 
Unio crprosus Barnes. 
Shell elongated, generally solid, rarely inflated, sometimes 
subcompressed, decidedly inequilateral, usually a little higher 
in front and often arcuate, especially in an adult state; beaks 
not much elevated above the curved dorsal line, generally sub- 
compressed, turned a little forward over a_ well-developed 
lunule, their sculpture a number of strong, often rude, sub- 
corrugated, longitudinal bars. which are sometimes slightly 
doubly looped; posterior ridge well developed, subangular or 
rounded, curved and placed close to the dorsal line, rarely dou- 
ble and ending behind in a point or biangulation at or near the 
base of the shell; surface with uneven growth lines, sometimes 
concentrically sculptured: epidermis dull, greenish or yellow- 
ish-brown in young shells, darker when old, often faintly raved 
in young specimens; left valve with two rather small, stumpy 
or subcompressed pseudocardinals; right valve with one, hav- 
ing occasionally a vestigial tooth in front of and behind it; 
laterals club-shaped, one in the right valve which is sometimes 
double, and two in the left ; beak cavities exceedingly shallow ; 
dorsal scars immediately under the hinge; muscle scars deep; 
pallial line impressed, crenate; nacre deep purple, salmon, 
straw-colored or white, obliquely ribbed. 
Length 145, height 68, diam. 45 mm. 
Length 125, height 56. diam. 26 mm. 
Length 123, height 65, diam. 29 mm. 
Length 109, height 47, diam. 30 mm. 
Entire Mississippi drainage; St. Lawrence and its tributar- 
ies; Alabama River system; southeast into Florida; southwest 
to the Guadalupe River. Texas. 
Type locality. Wisconsin. 
Unio nasuta L.AMARCK, An. sans Vert., VI, 18109, p. 75. 
Unio nasutus Acassiz, Arch. fiir Nat., I, 1852, p. 50. 
