PLATE 22, 23. 
other part of the shell, as long as the specimen remains 
whole. With the slightest blow from a hammer, however, 
it constantly divides, ivhere the edge of the smaller valve rests 
against the inside of the elongated, cylindric part of the larger 
valve; generally about half an inch, or less, from the 
top of the shell; (see fig. 1. b.) one side of the valve 
before hidden, then becomes visible ; sometimes in the 
smaller, but more generally in the larger division of the 
specimen, as delineated in fig. 2. 
Fig. 1. The most perfect form in which this petrifaction 
occurs, a. a. a. a rude mass of limestone filling the inner 
part of the shell, b. marks the line where the specimen 
divides. It is proper to observe, that no external appear- 
ance of any commissure is discoverable, previous to the 
breaking of the specimen. 
2. The same in a dilferent position, the top broken otf. 
3. Inside of the top, as separated from fig. 2. 
Amongst fifty specimens, or more, of this fossil, which 
I have examined, I have never met with one that did not 
break as described ; or when broken, did not exhibit the 
appearances delineated in the foregoing figures. The out- 
side of the specimens generally consists of a white, friable, 
spar. 
