NORTH-CAROLINA GEOLOGICAL SURVEY. 



281 



which do not reach the hinge or umbo ; many of the striae, 

 however, fork or divide. 



P. PEEDEENSIS. 



Shell thick and strong, broadly ovate ; ribs, eight, broad 

 striae, lines of growth strong towards the margin ; beak pro 

 jecting beyond the hinge line. 



Only one valve has been found of this species, and being 

 old and its striae obliterated in part, and its characters are 

 less distinct than is usual in specimens belonging to this ge 

 nus. 



p. MORTONI. 



Shell large, circular, compressed, thin, pearly ; equivalve 

 equilateral ; concentrically marked by fine lines of growth ; 

 on the outside, ribs are invisible ; inside, ornamented by about 

 eighteen pairs of ribs, which are prominent at the margin, 

 and obsolete towards the hinge. 



This beautiful shell occurs in the miocene at Waccamaw 

 Lake, North-Carolina, and has not been observed upon the 

 Xeuse or farther north. 



P. JEFFERSONIUS. (Fig. 199.) 



FIG. 199. 



