The Cainozoic beds both of New Zealand and southern Australia yield many species of _Dentalium_, the commonest and most widely distributed being the longitudinally ribbed _D. mantelli_ (Fig. 97 B), which ranges from the Balcombian to the Werrikooian stages in Australia, and is also typical of the Oamaru Series in New Zealand, where it is accompanied by the ponderous species, _D. giganteum_, which attained a length of over six inches. Another form common in our Cainozoics is the smooth-shelled _D. subfissura_; this also has a wide range, namely Balcombian to Kalimnan. =Palaeozoic Chitons.--= The _POLYPLACOPHORA_ or Chitons ("Mail-shells"), first appeared in the Ordovician. In Australia _Chelodes calceoloides_ (Fig. 97 C) is found in the Silurian of Derrengullen Creek, Yass, New South Wales; and another species of the genus is found in beds of the same age at Lilydale, Victoria. Between that period and the Cainozoic or Tertiary there is a gap in their history in Australia. =Cainozoic Chitons.--= _Ischnochiton granulosus_ (Fig. 97 D) is a Balcombian species of the modern type of "mail-shell," occurring not infrequently in the clays of Balcombe's Bay, Port Phillip, Victoria. _Cryptoplax pritchardi_ (Fig. 97 E) is a curious form belonging to the attenuated, worm-like group of the Cryptoplacidae, until lately unknown in the fossil state; it is found in the Kalimnan Series near Hamilton, Victoria. Several other genera of the chitons are found fossil in the Australian Cainozoics which still live on our coasts, as _Lorica_, _Plaxiphora_ and _Chiton_. The first-named genus is represented fossil by _Lorica duniana_ from the _Turritella_ bed (Janjukian) of Table Cape, Tasmania. =Characters of Gasteropoda.--= The _GASTEROPODA_ ("belly-foot") or univalve shells possess a muscular foot placed beneath the stomach and viscera. In the Heteropoda this foot is modified as a vertical fin, and in the Pteropoda as two wing-like swimming membranes close to the head. The mantle lobe is elevated along the back like a hood, and its surfaces and edges secrete the shell which contains the animal. The shell is typically a cone (example, _Patella_ or Limpet) which is often spirally coiled either in a plane (ex. _Planorbis_), conically turbinoid (ex. _Trochus_), or turreted (ex. _Turritella_). The body and shell are attached by muscles, the spiral forms being attached to the columella or axial pillar, and the bowl-shaped forms to the inner surface of the shell.