198 
PEOCEEDINGS OF THE PEETHSHIEE SOCIETY OF NATUEAL SCIENCE. 
are built up, and the simplest example we have of this is the 
common limpet. It is the consideration of the way in 
which a limpet constructs its conical shell that gives us the 
clue to the construction of the more complex spiral forrrs. 
In this mollusk the collar of the mantle adds to the margin 
of the shell equally all round, simply enlarging its radius as 
the creature increases in size. But supposing the front of 
the mantle were to be more active, and to build faster, so 
to speak, than the back, what would happen ? The front 
portion of the shell would increase at a greater rate than 
the back until it curled round upon the latter, and thus in 
time a flat spiral would be formed, such as the familiar 
ammonite. But let us suppose, further, that in addition 
to this the activity of ene side is in excess of that of the 
other. We have then two influences at work, and it is a 
mere matter of calculation to ascertain that the resultant 
of these two foices will be a spire not coiled flat, but more 
or less drawn out, or, in mathematical phrase, a spire 
whose plane is constantly changing. It is thus that the 
infinite varieties of spiral forms originate, varying from 
the nearly plane Harpa or Ampullaria to the tapering 
spire of Terebra or Mitra. Probably it is the 
position of the heart in the different species which 
determines this variation of form, as mollusks with reversed 
shells are found to have the position of the heart reversed 
also. If this be the true explanation, it affords a curious 
exam pie of the connection which is sometimes seen between 
the working out of a mathematical law and a purely 
physiological cause. Almost all spiral univalves have the 
spire turned from left to right, but there are some species, 
and even one or two genera, in which the reverse is the case. 
Of the former, however, it is by no means uncommon to 
find deformed specimens in which the shell has taken the 
opposite to the normal turn. This sometimes occurs with 
the common garden snail, and other land species. It is 
curious to note that while one species of whelk common on 
our coasts at the present day (Fusus anliquus ) has normally 
a dextral or right-turn shell, in a fossil state, as it occurs 
in certain deposits in the south of England, it has normally 
a sinistral or left-turn shell. The creature has, therefore, 
undergone in the course of ages a curious physiological 
moaification, so that what was formerly the exception is 
now the rule, and vice versa. 
Many spiral shells exhibit characteristic marking, such 
as raised plates or spines, which occur at regular intervals 
on each whorl. These mark periods of special activity of 
the mantle, occurring at definite intervals of time. The 
spiney murex is a good example of this; and if one of its 
spines be examined it will be found to consist of a hollow 
tube, not quite closed along one side, showing that it was 
formed within an extended fold of the mantle. In addi¬ 
tion, however, to these special growth-marks, which may 
be compared to the annual rings in a tree stem, all shells 
are more or less marked with what are called “ lines of 
growth,” which mark the steady progress of the building 
process, without indicating any definite period of rest or 
activity. 
The shell does not continue to grow during all the life 
of the mollusk, but after maturity has been reached it in¬ 
creases in thickness only, Most univalves, also, when 
they attain their full growth, have the lip greatly strength¬ 
ened by an outer rib, and sometimes ornamented as well by 
the addition of spines or other processes. A familiar ex¬ 
ample of the latter among our native mollusca is the 
“ pelican’s-foot ” shell (Aporrhais pes-pelicani), which de¬ 
rives its name from this peculiarity, while its gigantic tro¬ 
pical cousin, the Fteroceras, is merely an exaggerated 
example of the same. I have said that the mollusk thick¬ 
ens its shell after it has attained maturity, but a remark¬ 
able exception to this occurs in the case of one or two 
genera, which actually dissolve away part of the inner 
partition walls of their shells in order to increase their 
accommodation. This only occurs with forms such as 
Conus and Oliva, whose shells are so closely coiled as to 
leave very little space for expansion. The inner wall is 
not entirely removed, but is reduced to the thinness of 
paper. 
I find that the limits of this paper will not permit me to 
say anything about the microscopic structure of shells, 
which forms an interesting study in itself. There are 
other points also, especially in regard to the shells of 
bivalves and of cephalopods, to which I should like to have 
referred, but I fear I have taxed your patience too far 
already with what, to many, must appear dry details of 
structure. My only excuse is the unfailing pleasure which 
the study of these objects has always yielded me, and the 
desire to lead some to look upon them not as mere toys, 
but as among the most beautiful and wonderful pieces of 
workmanship in God’s world. It has sometimes been 
urged against the study of natural science that much of the 
charm and poetry of ^Nature is lost to the naturalist as he 
learns to look beneath the surface of the objects he sees 
around him, and to enquire into the how and wherefore of 
their existence; but I trust I have said enough to show 
that, in the case of shells at least, the pleasure of a ramble 
by the seaside will not be diminished but greatly enhanced 
by such a study, if entered into in the right spirit. 
