the porous and solid corals of the modern seas; but, in so far as we know, the animals producing these, though differing in some details, were neither more nor less elevated than their predecessors, and they took up precisely the same rôle as reef-builders in the sea, though with probably more tendency to the accumulation of great masses of coral limestone in particular spots. [Illustration: FIG. 43.—Tabulate Corals. _a_, _Halisites_, and _b_, _Favosites_. Upper Silurian.] [Illustration: FIG. 44.—Rugose Coral (_Heliophyllum Halli_). Devonian.] [Illustration: FIG. 44_a_.—_Zaphrentis prolifica_ (Billings). Devonian.] [Illustration: 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 sea-urchins. 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). [Illustration: FIG. 46.—Modern Crinoid (_Rhisocrinus Lofotensis_).—After Sars.] [Illustration: FIG. 47.—_Palæaster Niagarensis_ (Hall). One of the oldest star fishes.] [Illustration: FIG. 48.—_Palæchinus ellipticus_ (McCoy). One of the oldest types of sea-urchins.]