INSECTS.--Notwithstanding their fragility, insects are often well preserved as fossils, for the reason that their skin and wings consist of the horny substance called chitin. The Tertiary marls of Europe are very prolific in insect remains (Fig. 19). From the Miocene beds of Florissant, Colorado, U.S.A., several hundred species of insects have been described. [Illustration: =Fig. 18.= =A Fossil Fish with Ganoid Scales (Pristisomus crassus, A.S. Woodw.).= About 1/2 nat. size. Trias (Hawkesbury Series), of Gosford, New South Wales. (_Nat. Mus. Coll._) ] [Illustration: =Fig 19.--A Fossil Insect (Tipula sp.) in Amber.= Nat. size. Oligocene beds; Baltic Prussia. (_F.C. Coll._) ] [Illustration: =Fig. 20.--A Fossil Lobster (Thalassina emerii, Bell).= Slightly reduced. From the Pleistocene of Port Darwin, Northern Territory. (_Nat. Mus. Coll._) ] [Illustration: =Fig. 21.--An Ammonite (Desmoceras flindersi, McCoy sp.)= Half nat. size. Showing complex sutures. L. Cretaceous: Marathon, Flinders River, Queensland. (_Nat. Mus. Coll._) ] CRUSTACEA.--The outer crust, or exoskeleton, of these animals is often hard, being formed of a compound of carbonate and phosphate of lime on an organic, chitinous base. The earliest forms of this group were the trilobites, commencing in Cambrian times, and of which there is a good representative series in Australian rocks. Remains of crabs and lobsters are found in the various Cainozoic deposits in Australia (Fig. 20), and also in the Jurassic in other parts of the world. 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). [Illustration: =Fig. 22. Belemnites (Belemnites diptycha, McCoy).= 1/3 nat. size. Lower Cretaceous. Central South Australia. (_Nat. Mus. Coll._) ] [Illustration: =Fig. 23.--A Group of Lamp Shells (Magellania flavescens, Lam. sp.)= Attached to a Polyzoan. About 1/3 nat. size. Dredged from Westernport, Victoria. (_C.J. Gabriel Coll._) ] 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.