OPHIUROIDEA, or Brittle-stars.
Characters of Brittle-Stars.—
The Brittle-stars are frequently found at the present day cast up on the fine sandy beaches of the coast. They are easily distinguished from true starfishes by having a definite central disc, to which the arms are attached. The arms are used for locomotion and prehension, and have their grooves covered over with plates. The ossicles of the arms are moveable and controlled by muscles which enable them to be used as feet. The lower surface of the disc has a central arrangement of five rhomboidal sets of jaws, formed of modified ossicles, called the mouth frame, whilst the upper surface bears, between one set of arms, the madreporite or covering plate to the water vascular system, as in starfishes.
Fig. 78—Protaster brisingoides, Gregory.
Negative cast of the calcareous skeleton. Nat. size.
Silurian Sandstone, Flemington, Victoria.
(Nat. Mus. Coll.)
Silurian Brittle-Stars.—
The Brittle-stars in Australia first appear in the Silurian, but in England and Bohemia date back to the Ordovician. Protaster is the commonest genus, and is represented by P. brisingoides of the Melbournian stage of Silurian strata at Flemington ([Fig. 78]). It also occurs rarely in the Yeringian beds at Yering, both Victorian localities. A very ornamental form, Gregoriura spryi, occurs in the same division of the Silurian at South Yarra. In this fossil the delicate spines attached to the adambulacral ossicles are well preserved and form a marginal fringe to the arm ([Fig. 79]). Sturtzura is another Silurian genus, found in the Wenlock of England and in the Melbournian of Flemington, Victoria.