The diogenodonta have lateral and cardinal teeth upon a hinge-plate, but never more than two laterals and three cardinals in any one group, as in _Crassatellites_. The cyclodonta have extremely arched teeth, which curve out from under the beaks, as in _Cardium_. [Illustration: =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.--= 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.