Fig. 79—A Brittle-Star. (Gregoriura spryi, Chapm.)
Nat. size. From the Silurian Mudstone of South Yarra, Victoria.
(Nat. Mus. Coll.)
Cainozoic Brittle-Stars.—
From the Victorian Cainozoic beds, in the Lower Pliocene of Grange Burn, Hamilton, a vertebral ossicle of an ophiurian has been obtained, which has been provisionally referred to the genus Sigsbeia.
ECHINOIDEA, or Sea-urchins.
This group is an important one amongst Australian fossils, especially those of Cainozoic age.
Characters of Sea-urchins.—
Echinoids are animals enclosed in a spheroidal box or test composed of numerous calcareous plates, disposed geometrically as in the Starfishes, along five principal lines. The test in the living condition is more or less densely covered with spines. The mouth is on the under surface. The anus is either on the top of the test (dorso-central), or somewhere in the median line between the two lower ambulacra. The ambulacra (“a garden path”) are the rows of perforated plates on the upper (abactinal) surface sometimes extending to the lower surface, through which protrude the podia, which in Starfishes are situated in grooves on the lower surface.