In the basal Ordovician mudstone of Heathcote, Victoria, there is a bivalve which in some respects resembles a Modiolopsis (?M. knowsleyensis), but the exact relationship is still doubtful.
Silurian Bivalves.—
The Silurian sandstones, mudstones, slates and limestones of Australia and New Zealand, unlike the older rocks just mentioned, contain a rich assemblage of bivalve fossils. In Victoria the lower division or Melbournian stage contains the following principal genera:—Orthonota, Grammysia, Leptodomus, Edmondia, Cardiola, Ctenodonta, Nuculites, Nucula, Palaeoneilo, Conocardium, Modiolopsis and Paracyclas. The upper division or Yeringian stage contains other species of similar genera to those in the Melbournian, as Grammysia, Palaeoneilo and Conocardium; whilst Panenka, Mytilarca, Sphenotus, Actinodesma, Lunulicardium, Actinopteria and Cypricardinia are, so far as known, peculiar to this and a still higher stage. Cardiola is a widely distributed genus, occurring as well in Tasmania; whilst in Europe it is found both in Bohemia and Great Britain. Its time-range in the northern hemisphere is very extensive, being found in beds ranging from Upper Ordovician to Devonian. Actinopteria is found also in New South Wales and New Zealand, and Pterinea and Actinodesma in New South Wales.
The molluscs with a taxodont hinge-line (beset with numerous little teeth and sockets) are quite plentiful in the Australian Silurian; such as Nucula, a form common around Melbourne (N. melbournensis ([Fig. 90 D])); Nuculites, which has an internal radial buttress or clavicle separating the anterior muscle-scar from the shell-cavity, and which is found likewise in the Melbourne shales (N. maccoyianus ([Fig. 90 E])); Ctenodonta, represented in both the Melbournian and Yeringian stages (C. portlocki); and Palaeoneilo, a handsome, subrostrate generic type with concentric lamellae or striae, commonest in the Melbournian, but occasionally found in the younger stage (P. victoriae Fig. 90 F, Melbournian;—P. raricostae, Yeringian). Conocardium is represented by two species in Victoria (C. bellulum and C. costatum); whilst in New South Wales C. davidis is found at Oakey Creek. In New Zealand Actinopteria and Pterinea occur in the Wangapeka series (Silurian).
Devonian Bivalves.—
The compact limestone and some shales of Middle Devonian age in the N.E. Gippsland area in Victoria, contain several as yet undescribed species belonging to the genera Sphenotus, Actinodesma and Paracyclas.
A—Mytilarca acutirostris, Chapm. Silurian. Victoria
B—Modiolopsis melbournensis, Chapm. Silurian. Victoria
C—Goniophora australis, Chapm. Silurian. Victoria
D—Paracyclas siluricus, Chapm. Silurian. Victoria
E—Actinopteria australis, Dun. Devonian. New South Wales
F—Lyriopecten gracilis, Dun. Devonian. New South Wales
The genera Paracyclas, Aviculopecten and Pterinea have been recorded from New South Wales, chiefly from the Yass district. The derived boulders found in the Upper Cretaceous beds forming the opal-fields at White Cliffs, New South Wales, have been determined as of Devonian age. They contain, amongst other genera, examples of Actinopteria (A. australis), Lyriopecten (L. gracilis) ([Fig. 91 F]), and Leptodesma (L. inflatum and L. obesum).