qo THE GEELONG NATURALIST. 
SHELLS AND WHAT THEY TEACH. 
(Continued.) 
Bx J. F. MULDER. 
SuELnLFISH may be divided into four or five great classes, according 
to their mode of motion. Thus we have the Cuttlefish (cephalopoda), 
the Snails (Gasteropoda), the Pteropoda, the Conchifere and 
Brachiopoda, and the Lamellibranchiata. 
The antiquity of these types is immense. They make their 
first appearance in the Lower Silurian or Upper Cambrian, and . 
appear to have thrown out species and varieties in bewildering 
numbers. By a careful study of the different formations of the 
earth's crust we can see the time when each species makes its 
appearance, when it reaches its greatest development, and when it 
passes away. Some species live through only one geological forma- 
tion, like the Hippurites, which are only found in the cretaceous 
rocks. Some live through two or more, and some, like the Pectens, 
extend through a whole period. 
To illustrate the development and decay of different, species of 
mollusca, let us glance at some of our commoner shells. The Fusis. 
or Whelk grows in the South Seas up to thirty inches in length, 
while the fossil Fusis in the Eocene is only three inches long. The 
reverse is the case with the Cowrie (Cyprea) and Nautilus tribes. 
In the Eocené the Nautili were as big as small Crickets, and some 
of the Cypræae measured a foot in length. 
Sir Charles Lyell has taught us to regard the stratified rocks 
as so many monuments recording the physical condition and living 
inhabitants of the earth in past ages. 
The organic remains are changed gradually and regularly from 
the earliest period to the latest formations, so that the mass of 
species in each period must have been peculiar and distinctive. 
William Smith says: “ Fossils are to the naturalist what coins 
are to the antiquary.” According to M. M. Agassiz and D'Orbigny, 
nearly all the fossils of each formation are peculiar, very few 
species being supposed to have survived from one period to another. 
Sudden and entire changes of this kind only take place when 
the nature of the deposit is completely altered—as when sands or 
clays rest on chalk—and in these cases there is usually evidence, as. 
by change of dip, that an interval must have elapsed between the 
completion of the lower stratum and the commencement of the 
