MOLLUSCA. 
123 
carbonate of lime. Tlie fossil remains of the Mollusca consist 
therefore chiefly of shells, and these in substance and appear- 
ance may not differ from shells that one picks np in the 
fields or on the sea-shore. Such are many of the Post- 
Pliocene non-marine shells, and Neritina concava from the 
Oligocene of Headon. Usually, however, the horny layers 
of the shell have entirely disappeared, and are replaced by 
a secondary deposit of lime that has soaked in from the 
surrounding rock and hardened or petrified the shell. Some- 
times the original lime itself has been replaced by another 
mineral, such as Hint or iron pyrites, and sometimes the 
whole shell has disappeared, leaving only a cavity in the 
rock, with an imprint (or external cast) of the shell on its 
outer walls, and sometimes containing a mass of rock that 
filled the interior of the shell, and is called an internal cast. 
This last method of preservation is clearly shown in a large 
block from the Poach bed in the quarries of Portland, Dorset. 
Shells presei-ved in flint are common among fossils from the 
Chalk, and pyritised shells are generally found in clays of 
all ages. A few iMollusca possess other hard parts capable of 
fossilisation ; thus several of the cuttle-fish and their allies 
have horny beaks, which form the fossils called Ehyncholithes 
(beak-stones. Fig. 78), some have horny hooks on their arms, 
and some, buried in clay, have even left traces of skin, 
muscles, and eyes (Fig. 86). We can also infer the former 
presence of certain boring molluscs, such as Pholas, by the 
burrows they have made in the rocks of the old sea-floor. 
Mollusca are in some of their forms, such as oysters and 
snails, so familiar that we need here only recall the fact that 
they are among the more highly organised invertebrates, 
having in the simpler types a distinct hecud, a mouth and 
complete digestive system, and a thickening on the under 
side of the body forming a muscular organ ^dled the foot. 
All molluscs have a nervous system, blood-vessels, a heart, 
gills of varying origin, and excretory and reproductive organs. 
Further information on these matters may be sought in the 
Zoological Department. 
The shells of molluscs are built on various plans ; the 
oyster, for instance, has a shell of two valves ; the whelk or 
the snail-shell is all in one piece, and is besides coiled and 
open only at one end ; the nautilus shell is also coiled, but 
when cut through it is seen to be divided up by partitions 
into a number of chambers ; the tooth-shell, Dentalium, is 
a tube shaped like a tusk, but open at both ends ; Chiton, 
Gallery 
VIII. 
Upright 
Case Al. 
Table-case 
3 . 
Centre of 
Gallery 
VIII. 
Gallery 
VII. 
Table-case 
I. 
Gallery 
VIII. 
Table-case 
9 . 
