' MOLLUSCA. 241 
criminated by the petrifactions which they contain. From 
the difficulty of distinguishing the fossil species, however, 
joined with our ignorance of their geographical distribution, 
some mineralogists have not permitted their conclusions to 
be much influenced by this rule. 
It will likewise be observed, that the shells in the newer 
strata are but little changed, whilst those in the older rocks 
are greatly altered in their texture, and in part obliterated. 
The same power which rendered the rock compact or crys- 
talline, has likewise exerted its influence on the imbedded 
remains. In the newest strata, this power has scarcely be- 
gun to operate; so that the imbedded shells still retain in 
perfection their original characters. 
In examining a limestone quarry, for example, the stu- 
dent will perhaps be surprised to find petrifactions of shells 
in the bed of limestone, while, in the sandstone covering, 
he witnesses impressions of plants unaccompanied with 
shells. In order to gain more correct ideas on this subject, 
let him repair to a marl bog, and he will there find the bed 
of marl abounding in shells, while in the bed of sand below, 
on which it rests, or of peat moss, which covers it, he will 
find exclusively the remains of vegetables. Here let him 
study the subject, while the strata are yet recent, and while 
lapidification is only commencing. There is, however, this 
difference between the shells in the marl and those in the 
limestone, that individuals of the former species still exist, - 
while no living examples of the latter are known. 
° Gerocraruicat History or Foss SuHeiis.—As the 
geographical distribution of recent shells is a branch of con- 
chology to which few have devoted their attention, and about 
which very little is known, we can scarcely expect to find the 
geographical distribution of the fossil species more fully illus- 
M 
