MOLLUSCA.—The Cuttle-fish group (Cephalopoda, “head-footed”), is well represented by the Nautilus-like, but straight Orthoceras shells commencing in Ordovician times, and, in later periods, by the beautiful, coiled Ammonites ([Fig. 21]). The true cuttle-fishes possess an internal bone, the sepiostaire, which one may see at the present day drifted on to the sand at high-water mark on the sea-shore. The rod-like Belemnites are of this nature, and occur abundantly in the Australian Cretaceous rocks of South Australia and Queensland ([Fig. 22]).
Elephant-tusk shells (Scaphopoda) are frequent in our Tertiary beds: they are also sparingly found in the Cretaceous, and some doubtful remains occur in the Palaeozoic strata of Australia.
The shells of the ordinary mollusca, such as the snails, whelks, mussels, and scallops, are abundant in almost all geological strata from the earliest periods. Their calcareous shells form a covering which, after the decay of the animal within, are from their nature among the most easily preserved of fossil remains. There is hardly an estuary bed, lake-deposit, or sea-bottom, but contains a more or less abundant assemblage of these shell-fish remains, or testacea as they were formerly called (“testa,” a shell or potsherd). We see, therefore, the importance of this group of fossils for purposes of comparison of one fauna with another (antea, [Fig. 1]).
The chitons or mail-shells, by their jointed nature, consisting of a series of pent-roof-shaped valves united by ligamental tissue, are nearly always represented in the fossil state by separate valves. Fossil examples of this group occur in Australia both in Palaeozoic rocks and, more numerously, in the Cainozoic series.
MOLLUSCOIDEA.—The Brachiopods or Lamp-shells consist generally of two calcareous valves as in the true mollusca ([Fig. 23]), but are sometimes of horny texture. Like the previous class, they are also easily preserved as fossils. They possess bent, loop-like or spiral arms, called brachia, and by the movement of fine ciliated (hair-like) processes on their outer edges conduct small food particles to the mouth. The brachia are supported by shelly processes, to which are attached, in the Spirifers, delicate spirally coiled ribbons. These internal structures are often beautifully preserved, even though they are so delicate, from the fact that on the death of the animal the commissure or opening round the valves is so tightly closed as to prevent the coarse mud from penetrating while permitting the finer silt, and more rarely mineral matter in solution, to pass, and subsequently to be deposited within the cavity. At the Murray River cliffs in South Australia, a bed of Cainozoic limestone contains many of these brachiopod shells in a unique condition, for the hollow valves have been filled in with a clear crystal of selenite or gypsum, through which may be seen the loop or brachial support preserved in its entirety.
The Sea-mats or Polyzoa, represented by Retepora (the Lace-coral) ([Fig. 24]) and Flustra (the Sea-mat) of the present sea-shore, have a calcareous skeleton, or zoarium, which is easily preserved as a fossil. Polyzoa are very abundant in the Cainozoic beds of Australia, New Zealand, and elsewhere ([Fig. 25]). In the Mesozoic series, on the other hand, they are not so well represented; but in Europe and North America they play an important part in forming the Cretaceous and some Jurassic strata by the abundance of their remains.