240 MOLLUSCA. 
and, in many cases, intimately united with the substance of 
the beds; the shells belonging to the newer members are 
much less altered in their form and texture, separate more 
readily from the surrounding rocks, and appear like recent 
shells somewhat weathered. The shells are found in nearly 
all the different kinds of rock, but are more numerous in 
the calcareous strata. In the recent or superficial strata, 
fossil shells are frequently to be met with. The species 
which here present themselves bear so close a resemblance 
to the existing kinds, that conchologists are disposed to con- 
sider them as the relics of animals which still exist. In 
many cases, the prototypes may be found on the neighbour- 
ing shore or lake, but in other instances they must be sought 
for at a greater distance. ‘These shells are found in beds 
of gravel and sand, and likewise in great abundance in shell 
marl. 
It appears, then, that the shells in the older strata differ 
specifically from those which the newer strata contain; and 
that they have belonged to molluscous animals, which no 
longer exist in a living state on this globe; that, in the 
newer strata, the fossil shells bear a closer resemblance to 
existing species; and that in the last formed strata, remains 
of species actually existing are to be met with. 
In this geological distribution of the remains of testace- 
ous animals we may likewise perceive that, in the older 
strata, the inequivalved shells are more numerous than the 
other kinds; and that the canaliculated univalves are seldom, 
if ever, to be met with in the transition or older members 
of the floetz series, but that they become more numerous 
in the newer members of the floetz rocks, and in the allu- 
vial strata. Circumstances of this kind have induced geo- 
logists to conclude that different formations could be dis- 
