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.
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.