MOLLUSCA. 
Mollusea. the shell. The fossil species found in the rocks of the older 
<= —~ members of the class are greatly altered in their texture, 
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- 
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. Jn 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. 
GeocrarnicaL History or Fossin Surtis.—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- 
trated. We know, with regard to the recent shells, that 
some species which are found in the bays of Norway and 
Greenland occur also on the shores of the Mediterranean, 
and that the British Isles have several species in common 
VOL, XY. 
with Africa and the West Indies. 
377 
Still we know not, with Mollusca. 
any degree of accuracy, the geographical range of any one =~ 
species. Geologists ought, therefore, to exercise a great 
degree of caution in drawing conclusions concerning the 
original situation of those shells which they find in a fossil 
state. When a fossil shell is discovered in the strata of this 
country, which bears a close resemblance to the recent shells 
of distant seas; many inquirers, without waiting until they 
have established the identity of the species, and without any 
precise information with regard to the geographical distri- 
bution of that species, conclude that this fossil shell must 
have been brought from these distant seas, and conveyed to 
its present situation by some mighty torrent. Instances of 
this mode of reasoning could easily be pointed out in the 
writings of British and Continental mineralogists. 
In every country there are particular animals and vege- 
tables, which indicate, by their mode of growth and rapid 
increase, a peculiar adaptation to the soil and climate of 
that district. Hence we find aremarkable difference in the 
animals and plants of different countries. Many shell-fish 
have indeed a very wide range of latitude, through which 
they may be observed; but we know, that the same mol- 
luscous animals which are natives of Britain, are not found, 
as a whole, as natives of Spain, while the molluscous ani- 
mals of Africa differ from both. If the same arrangement 
of the molluscous animals always prevailed in the different 
stages of their existence, then we may expect to find the 
fossil shells of one country differing as much from those of 
another, as the recent kinds are known to do, so that every 
country will have its fosse/, as well as its recent testacea. 
Few observations illustrative of this branch of the subject 
have hitherto been published. 
It has often been remarked, that the fossil shells (and the 
relics of other animals and plants) found in the strata of this 
country, are very different in their appearance from those 
shells of the mollusca which at present exist in the country, 
but that they bear a close resemblance to the existing 
species of the equatorial regions. This very important ob- 
servation has led some to conclude, that the mollusca which 
lived in this country at the period of the formation of the 
strata in which they are now enclosed, were influenced by 
different physical circumstances, from those by which the 
forms of the recent kinds are regulated ; while others have 
imagined that those shells once lived in the equatorial re- 
gions, and that a mighty deluge transported them to their 
present situation. This last conclusion can never be ad- 
mitted by those who have witnessed the perfect preservation 
of the different parts of fossil shells, their valves, spires, 
protuberances, and delicate spines, still unbroken. Though 
these species no longer exist in a living state in this coun- 
try, nor on the globe, we must admit the conclusion of 
Werner, with regard to fossil plants, that they lived and 
died in the country where their relics are now found. 
It would form a very curious subject of inquiry to ascer- 
tain the character of those fossil shells which are found in 
the strata near the equator. If they likewise differ from 
the recent species of those seas, and if, in appearance, they 
resemble or differ from the productions of arctic regions, we 
might then speculate, with more success, upon those mighty 
revolutions which have taken place on the earth’s surface, 
and trace in the mineral kingdom the proofs of those changes 
which animals and vegetables have experienced. In the 
meantime, we would recommend the examination of the 
laws which regulate the physical and geographical distribu- 
tion of recent shells and molluscous animals, as the most 
suitable preparation for investigating the condition of those 
extinct races, the memorials of which are preserved in stra- 
ta, differing from one another in structure, in position, and 
in composition. 
3B 
