Gasteropod shells are normally right-handed (dextral), but a few genera as _Clausilia_, _Bulinus_ and _Physa_, are left-handed (sinistral). The height or length of the shell is measured from the apex to the lower margin of the mouth. In coiled shells we may regard them as a more or less elongated cone wound round a central pillar, the columella, or around a central tube. A turn or coil of the shell is a whorl, and together, with the exception of the last, form the spire. The line between two adjacent whorls is the suture. When the columella is solid the shell is said to be imperforate, and when a central tube is left by the imperfect fusion of the whorls, it is perforate. The opening of the tubular columella is termed the umbilicus, and this is sometimes contracted by the encroachment of shell matter termed the callus. The aperture is entire when the rim is uninterrupted; and channelled when there is a basal notch, where the siphon which conducts water to the gills is lodged. As a rule the large heavy gasteropods inhabit shallow water. The following living genera are characteristic of rocky shore-lines; _Risella_, _Buccinum_, _Purpura_ and _Patella_. Genera typical of sandy shores are _Nassa_, _Natica_, _Cypraea_, _Turritella_ and _Scala_. =Cambrian Gasteropods.--= From the Cambrian of South Australia Prof. Tate described some minute Gasteropods which he referred to the genera _Stenotheca_ (_S. rugosa_, var. _paupera_), _Ophileta (O. subangulata)_ (Fig. 98 A), and _Platyceras (P. etheridgei)_. In these beds at Curramulka the following Pteropods were found by the same authority, viz., _Salterella planoconvexa_, _Hyolithes communis_ (Fig. 98 C) and _H. conularioides_. The Cambrian Limestone of the Kimberley District, W. Australia, contains the characteristic Pteropod _Salterella hardmani_ (Fig. 98 B). The shell is a conical tube, straight or slightly curved, and measuring scarcely an inch in length. [Illustration: =Fig. 98--LOWER PALAEOZOIC GASTEROPODA.= A--Ophileta subangulata, Tate. Cambrian. South Australia B--Salterella hardmani, Foord. Cambrian. West Australia C--Hyolithes communis, Billings. Cambrian. South Australia D--Scenella tenuistriata, Chapm. Cambrian. Victoria E--Raphistoma browni, Eth. fil. Ordovician. South Australia F--Helicotoma johnstoni, Eth. fil. Silurian. Tasmania ] The Upper Cambrian of the Mersey River District in Tasmania has afforded some doubtful examples of the genus _Ophileta_. In the Upper Cambrian Limestones of the Dolodrook Valley, near Mt. Wellington, Victoria, a minute limpet shaped Gasteropod occurs, named _Scenella tenuistriata_ (Fig. 98 D). =Ordovician Gasteropods.--=