Review of the New York Geological Reports. 67 
almost imperceptible gradations into a species, with a wide cardinal 
area which cannot be distinguished from J. macronota. If we 
are not mistaken, this is the fossil described by Conrad under the 
name D. duplicata. It is an abundant fossil also in the Louis- 
‘pysaduor siuhynag *% Biq 
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ville water-limestone. We should not be surprised if more ex- 
tended observation might prove D. medialis, fig. 8 of the following 
wood-cut, to be only a modified form of the same. ‘The number of 
ribs, and laminw of growth, and width of cardinal area, are char- 
acters which certainly are liable to variation in the same species. 
Acuteness of the mesial fold, too, depends considerably upon age 
and condition. 
