284 Dr. Hildreth on fresh water Shells. 



Length, 2.00 ; breadth, 3.5 ; diameter, 1.4. — Hab. Musk- 

 ingum. 



Shell, thick and ponderous ; anterior side, narrowed, thin, 

 angulated ; beaks, low ; anterior lunule, carinated ; basal 

 margin, arcuated ; anterior margin, narrow and rounded ; 

 dorsal margin, higher than the beaks ; posterior margin, 

 rounded and slightly gaping ; epidermis, light olive and fine- 

 ly wrinkled transversely ; cardinal teeth, rather small, lightly 

 sulcated, and finely crenated ; lateral teeth, very broad and 

 thick ; posterior muscular impression rough and deep ; an- 

 terior one, deep and striated ; naker, pearly ; cavity of the 

 beaks, shallow, and inner surface marked with several deep 

 folds, running obliquely from the cardinal teeth to the ante- 

 rior margin. 



Remark. — Quite a common shell in the Muskingum. 



No. 15. Unio Orbiculatus. — Fig. 15. 



Shell, nearly round ; inflated, beaks somewhat promi- 

 nent, broad and directed backwards ; anterior lunule, broad 

 heart shaped ; cardinal teeth, elevated, angulated. 



Length, 2.5 ; breadth, 2.5; diameter, 1.75. — Hab. Musk- 

 ingum. 



Shell, nearly orbicular ; anterior margin, broad, and slight- 

 ly rounded ; posterior, short and narrow ; disks, much in- 

 flated ; dorsal margin, lightly rounded, and basal margin the 

 same ; ligament, thick and elevated, passing between the 

 beaks ; beaks, a little projecting, distant and decorticated ; 

 epidermis, a dark chesnut on the center of the disks, pas^ 

 sing into a light brown as it approaches the margin ; surface 

 lightly waved on the upper part of the disks, and finely wrin- 

 kled below, transversely ; cardinal teeth, direct, elevated 

 and deeply sulcated; lateral teeth, thick and prominent; 

 posterior cicatrix, deep, and rough before ; anterior cicatrix, 

 broad, finely waved ; striated and beautifully iridescent ; ca- 

 vity, broad and deep ; naker, flesh color, and very iridescent 

 with purple and violet. 



Remarks. — This shell is a variety of the crassus ; but dif- 

 fers so much from any I have seen that it deserves notice. 



No. 16. Unio Foliatus, Fig. 16. 

 Shell, shaped like a grape leaf, surface waved ; disks, 

 swelled ; base, arcuated, and anterior margin deeply emar- 

 ginate. 



