A Deceptive FossiUzatjon. — Sardcsoii. 
41. 
produced. I think, by thick shells buried or burrowed into 
sediment, which were dissolved away leaving merely a black 
stani in place of a shell, while the imperfectly consolidated 
matrix closed then into space evacuated by the shell. The cast 
and mold compressed one upon the other and hence each re- 
ceived some imprint of the other. The new black film forms 
a cleavage plane which the frost, much better than the geolo- 
gist's hammer, may open. 
The composition of the shell of this species was evidently 
different from that of associated pelecypods since the latter are 
differently fossilized, occurring abundantly as casts and molds, 
to be sure, but having open dividing spaces where the shell 
formerly iay, showing that the shells resisted absorption and 
held until the sediment around them had set. 
3- 
7 — s 
Description of Figures of Jllodiolopsis plana 11. 
A laterally compressed imprint. 
A vertically compressed specimen. 
A longitudinally shortened specimen. 
