The cyclodonta have extremely arched teeth, which curve out from under the beaks, as in Cardium.
Fig. 90—LOWER PALAEOZOIC BIVALVES.
A—Ambonychia macroptera, Tate. Cambrian. S. Australia
B—Grammysia cuneiformis, Eth. fil. Silurian. Victoria
C—Panenka gippslandica, McCoy sp. Silurian. Victoria
D—Nucula melbournensis, Chapm. Silurian. Victoria
E—Nuculites maccoyianus. Chapm. Silurian. Victoria
F—Palaeoneilo victoriae, Chapm. Silurian. Victoria
The teleodonts include the more highly developed types of hinge, with attenuated teeth and sockets. Common shells of our coast, and from Cainozoic beds, belonging to this group are Venus, Mactra and Meretrix.
The asthenodonta are boring and burrowing molluscs that have lost the hinge dentition from disuse as Corbula and Pholas.
Cambrian Bivalve.—
The earliest example of a bivalved shell in Australian rocks is Ambonychia macroptera ([Fig. 90 A]), which occurs in the Cambrian Limestone of Curramulka, S. Australia. It is quite a small form, being less than a quarter of an inch in length.
Ordovician Bivalve.—