$76 
MOLLUSCA. 
Mollusca. piece of gauze spread over a ring attached to the end ofa 
=" staff, forms a very convenient net for fishing fresh water 
shells. By means of this net in the drought of summer, al- 
most all the different species of fresh water shells may be 
obtained with ease. 
The fresh water shells are frequently covered over with 
slime or mud, which must be removed bya brush ; and the 
animal may be extracted after the shell has been plunged in 
boiling water. 
Before closing our remarks on the important group of 
animals to which we have been directing the attention of 
the reader, we shall dedicate a few paragraphs to a brief 
notice respecting Fossiz SHELLS. 
Besides the shells which are found on the land and in our 
lakes, rivers and seas, and termed Recent SHELLS, there are 
relics of many species found in our mar] pits and limestone 
rocks, always somewhat altered, and which are denominated 
Fossm Suexrs. While the shells of the former class have been 
eagerly sought after, few conchologists, previous to the be- 
ginning of the present century, directed their attention to 
the condition and distribution of the fossil species. Nearly 
six hundred species of recent shells have been described as 
natives of Britain, while the fossil species furnished by the 
strata of the different formations, and which have been ac- 
curately described, fall greatly short of that number. There 
is, however, reason to believe that the fossil species are even 
more numerous than the recent ones. 
It would have been a pleasant task for us to have entered 
into the details of this most important subject, but our limits 
permit us only to trace its outlines. Our remarks, however, 
we trust, will prove useful to those who are entering this 
fruitful field of investigation, and will embrace some obser- 
vations on the systematic characters, condition, situation, 
and distribution of these organic remains. 
Systematican History or Fosstn SHeLis.—The de- 
termination of the characters of fossil shells is attended with 
no inconsiderable amount of difficulty. The changes which 
they have undergone, and their union, in many cases, with 
the substance of the rock, having become incorporated with 
it, prevent us from ascertaining, withany degree of accuracy, 
the peculiar marks by which the species can be character- 
ised. No trace of the animal remains to aid us in the inves- 
tigation, so that all our distinctions must depend upon the 
eharacters furnished by the shell. This circumstance should 
prevent us from placing much confidence on the conclusions 
which have been drawn with respect to the resemblance be- 
tween fossil species, and those which still exist in a living 
state: 
The difficulty of determining the fossil species, and the 
reluctance to form new genera, rendered the descriptions 
of the older writers nearly unintelligible, although their 
figures are still useful to refer to. Lamark, aware of the 
imperfection of the characters of the genera of recent shells, 
as connected with this subject, and possessing a rich cabinet 
of the fossil species found in the neighbourhood of Paris, 
devoted much time to the illustration of this subject, and 
with great success, as his-various papers published in the 
Annales du Museum, abundantly testify. In this country, 
Parkinson, in his work entitled Organic Remains of a For- 
mer World, has added some important illustrations of the 
genera of Lamark, and has given some good descriptions of 
the species found in our rocks. Mr. Sowerby, in his Jéine- 
ral Conchology, (published in numbers), has given excel- 
lent figures of the British fossil shells; but we regret to 
add, that he has displayed too great anxiety to constitute 
species; and that the rocks in which they are found imbed- 
ed are but imperfectly characterised. But as the figures 
are well executed, they will prove highly useful to the Bri- 
tish mineralogist, by enabling him to refer to them with 
confidence, and to give names to those species which he 
meets with in the course of his investigations. 
&y 
Cupnican History or Foss Sariis.—When we con- a : 
sider the elements of which shells are composed, and the na- 
ture of their combination, we might be ready to expect that 
fossil shells would differ but little in structure from recent spe- 
cies. But the case is widely different. In many instances 
the confused foliaceous structure which prevailed in the re- 
cent shell, has given place toa new arrangement of the par- 
ticles, and the fossil shell exhibits a foliated crystalline struc- 
ture. Here solution and precipitation have taken place in 
the same spot, or the results have been effected by the slow 
operation of the corpuscular forces. In some cases the cal- 
careous matter of the shell has become impregnated with 
foreign ingredients, or has totally disappeared, leaving in 
its place ferruginous or siliceous depositions. But the most 
curious circumstance in the chemical history of these fos- 
sils, is the preservation of the animal matter of the shell in 
its origmal form and order of arrangement, even when the 
calcareous matter of the shell has been changed into com- 
pact or granular limestone. This very important fact we 
owe to the ingenuity of Mr. Parkinson, who, by treating 
the shell for a length of time with greatly diluted acid, ab- 
stracted the calcareous matter, and obtained a distinct view 
of the cartilaginous membranes of the shell. -The student 
will in general observe, that the cavities of those shells, 
which present an external opening, are filled with the same 
sort of matter as the rock in which they are enclosed, while 
the cavities, of the multilocular testacea, for example, which 
have no external communication, are filled-with matter in- 
variably of a crystalline structure, even when not different 
from the substance of the rock. 
Grocnostic History or Fossrn Sueius.—It appears 
evident that the advancement of this branch of conchology 
must, in a great measure, depend on the accurate discrimina- 
tion of the fossil species, and the relations of therocksin which 
they are contained. It is only within the last twenty years, 
therefore, that our knowledge of this branch of the subject 
has been acquired. The members of the Wernerian and 
geological societies have contributed largely to our stock of 
knowledge: but much yet remains to be brought to light. 
The following noticesmay be regarded as embracing the prin- 
cipal facts which have been ascertained. 
In those ancient strata upon which all the others are in- 
cumbent, and which are called primitive, no remains of 
shells, or other relics of organized bodies, have hitherto beer 
detected. These rocks are therefore supposed to have re- 
ceived their arrangement previous to the creation of animals 
and vegetables, or to have been so much altered as to have 
all traces of crganisms obliterated if such existed. In that 
group of rocks which rests upon the primitive strata, and to 
which mineraiogists give the name of transition, fossil shells, 
as well as the remains of vegetables, have been observed. 
The shells exhibit such striking peculiarities of form, and 
bear so remote a resemblance to the recent kinds, that they 
are considered as the remains of species which do not now 
exist in a living state on'the globe. They-aremuch chang- 
ed in their texture, and in general intimately united with 
the contents of the stratum. They are chiefly found inthe 
beds of limestone, sometimes also in the greywacke and clay 
slate. In the numerous and ill-characterised series of strata 
which are incumbent on the transition class, and to which 
some mineralogists attach the term jfoeéz, the remains of 
shells are much more numerous. In the older members of 
this class, such as the red sandstone and independent coal 
formations, the shells, though in a few instances different in 
form from those of the preceding class, appear to have be- 
longed to oneepoch. Theyare dissimilar tothe recent species, 
and no longer exist ina living state. In the newer members 
of this class, such as the gypsum and chalk rocks, the species, 
insome examples, bearamuch closer resemblance to the exist- 
ing races, and several species cannot be distinguished, itis al- 
leged, from them, by any satisfactory characters furnished by 
