MOLLUSCA. 



123 



carbonate of lime. The fossil remains of the Mollusca consist G-allery 

 therefore chiefly of shells, and these in substance and appear- VIII. 

 ance may not differ from shells that one picks up in the 

 fields or on the sea-shore. Such are many of the Post- Upright 

 Pliocene non-marine shells, and Neritina concava from the 

 Oligocene of Headon. Usually, however, the horny layers Table-ease 

 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 times the original lime itself has been replaced by another 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 fiint 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 Eoach bed in the quarries of Portland, Dorset. Centre of 

 Shells preserved in flint are com^mon among fossils from the ^vin'^ 

 Chalk, and pyritised shells are generally found in clays of 

 all ages. A few Mollusca possess other hard parts capable of 

 fossilisation ; thus several of the cuttle-fish and their allies Gallery 

 have horny beaks, which form the fossils called Bhyncholithes Table-case 

 (beak-stones. Fig. 78), some have horny hooks on their arms, i. 

 and some, buried in clay, have even left traces of skin, 

 muscles, and eyes (Fig. 86). We can also infer the former G-allery 

 presence of certain boring molluscs, such as Pliolas, by the TaMe-case 

 burrows they have made in the rocks of the old sea-floor. 9. 



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 head, a mouth and 

 complete digestive system, and a thickening on the under 

 side of the body forming a muscular organ called 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, Dentalmm, is 

 a tube shaped like a tusk, but open at both ends ; Chiton, 



