Fig. 44a.—Zaphrentis prolifica (Billings). Devonian.
Fig. 45.—Rugose Corals.
a, Zaphrentis Minas (Dn.), and b, Cyathophyllum Billingsi (Dn.). Carboniferous.
Leaving the corals, we may turn to the sea-stars and seaurchins. These merely put in an appearance in the Early Cambrian, but become vastly multiplied in the Silurian, where the stalked feather stars (Crinoids) ([Fig. 46]) seem to have covered great areas of sea-bottom, and multiplied so rapidly that thick sheets of limestone are largely made up of the fragments of their skeletons. The ordinary star-fishes appear first in the Silurian ([Fig. 47]). The sea-urchins begin in the Upper Silurian, the early species having numerous and loosely attached plates, like some of those now found in the deep sea[16] ([Fig. 48]).
Fig. 46.—Modern Crinoid (Rhisocrinus Lofotensis).—After Sars.