Cirripedes may be roughly divided into two groups, the Acorn Barnacles and the Goose Barnacles. Although dissimilar in general appearance, they pass through identical stages, and are closely related in most of their essential characters. The latter forms are affixed by a chitinous stalk or peduncle, whilst the acorn barnacles are more or less conical and affixed by the base.
Silurian Cirripedes.—
The stalked barnacles are probably the oldest group, being found as far back as the Ordovician period. In Australia the genus Turrilepas occurs in Silurian rocks, T. mitchelli ([Fig. 115 A]) being found at Bowning in the Yass District of New South Wales. The isolated plume-like plates of T. yeringiae ([Fig. 115 B]) are not uncommon in the olive mudstone of the Lilydale District in Victoria.
A—Turrilepas mitchelli, Eth. fil. Silurian. New South Wales
B—Turrilepas yeringiae, Chapm. Silurian. Victoria
C—(?) Pollicipes aucklandicus, Hector sp. Cainozoic (Oamaru series). New Zealand
Fig. 116—LIVING AND FOSSIL CIRRIPEDES.
A—Lepas anatifera, L. Common Goose Barnacle. Living
B—Lepas pritchardi, Hall. Cainozoic. Victoria