(obsolete) A taxonomic genus within the familyPteriidae — marinebivalves, having a pearly interior, allied to the pearl oyster; so called from a supposed resemblance of the typical species to a bird, the species of which are now in genera Pteria, Electroma, Pinctada.
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Rudimentary cephalic eyes occur in the Mytilidae and in Avicula at the base of the first filament of the inner gill, each consisting of a I pigmented epithelial fossa containing a cuticular lens.
The fossils from the Rhaetic beds belong to the Avicula contorta zone, those from the Lias to the Ammonites angulatus zone, while the blocks of limestone with chert contain Inoceramus, Cretaceous foraminifera and other organisms. The materials yielding these fossils are embedded in a course volcanic agglomerate which gives rise to crags and is pierced by acid and basic igneous ricks.