The highest class of the mollusca is the _CEPHALOPODA_ ("head-feet"). In these shell-fish the extremity of the body or foot is modified, and furnished with eyes, a funnel and tentacles. It has also strong horny beaks or jaws which make it a formidable enemy to the surrounding life in the sea. In the chambered forms of this group the animal partitions off its shell at regular intervals, like the Pearly Nautilus and the Ammonite, inhabiting only the last chamber cavity, but still communicating with the earlier series by a continuous spiral tube (siphuncle). In some forms like the living squid and the extinct Belemnite, the shell is internal and either spoon-shaped, or dart-shaped, that is, subcylindrical and pointed. =Characters of Cephalopod Shells.--Nautiloidea.--= In geological times the nautiloid forms were the first to appear (in the Ordovician), and they were either straight shells, as _Orthoceras_, or only slightly curved, as _Cyrtoceras_. Later on they became more closely coiled, and as they were thus less likely to be damaged, they gradually replaced the straight forms. The Ammonites have the siphuncle close to the outside of the shell, whilst in the Nautilus it is more or less median. The sutures or edges of the septa in _Nautilus_ and its allies are curved or wavy, but not so sharply flexed or foliaceous as in _Ammonites_. The Nautiloidea range from the Ordovician and are still found living. =Ammonoidea.--= The Ammonoidea appear in Devonian times and die out in the Cretaceous. They were very abundant in Jurassic times, especially in Europe. =Belemnoidea.--= The Belemnoidea, ranging from the Trias to Eocene, comprise the extinct _Belemnites_, the interesting genus _Spirulirostra_ of Miocene times, and the living _Spirula_. =Sepioidea.--= The Sepioidea or true Cuttle-fishes ("pen-and-ink fish") range from the Trias to the present day. =Octopoda.--= The Octopoda, with _Octopus_ and _Argonauta_ (the paper "Nautilus") are present-day modifications. The male of the latter is without a shell, the female only being provided with a delicate boat-shaped shell secreted by the mantle and the two fin-like expansions of the dorsal arms. =Ordovician Cephalopods.--= The Ordovician cephalopods of Australasia are not numerous, and are, so far as known, practically restricted to the limestones of the Larapintine series at Laurie's Creek and Tempe Downs, in Central South Australia. Amongst them may be mentioned _Endoceras warburtoni_ (Fig. 105 A), (a straight form in which the siphuncle is partially filled with organic deposits); _Orthoceras gossei_; _O. ibiciforme_; _Trochoceras reticostatum_ (a coiled form); and _Actinoceras tatei_ (a genus characterised by swollen siphuncular beads between the septa). [Illustration: =Fig. 105--PALAEOZOIC CEPHALOPODA.= A--Endoceras warburtoni, Eth. fil. Ordovician. South Australia B--Orthoceras lineare, Münster sp. Silurian (Yer.) Victoria