which were possibly also the cells of Hydroids, that have a similar history. They are the only known Corals that date so far back as the Upper Cambrian; and they continue under very similar forms all through the Palæozic, and are represented by the millepore corals of the present day. Fig. 40 represents a form found at the base of the Siluro-Cambrian, and Fig. 41 shows forms characteristic of the Carboniferous Limestone. [Illustration: FIG. 39_a_.—_Fenestella Lyelli_ (Dawson). A Carboniferous Bryozoan.] [Illustration: FIG. 40.—_Chaetetes fibrosa_. A tubulate coral with microscopic cells. Siluro-Cambrian.] If we turn now to the sea-mosses (Bryozoa), we have a group of minute polyp-like animals inhabiting cells not unlike those of the Hydroids, and which form plant-like aggregates. But the animals themselves are so different in structure that they are considered to be nearer allies of the bivalve shell-fishes than of the Corals. They are, in short, so different, that the most ardent evolutionist would scarcely hold a community of origin between them and such creatures as the Graptolites and Millepores, though an ordinary observer might readily confound the one with the other. These animals appear at the beginning of the Siluro-Cambrian, and such forms as that represented in Fig. 39, very closely allied to some now living, are large constituents of some of the limestones of that period. Other forms, like that represented in Fig. 39_a_, are very characteristic of the Carboniferous. These animals, individually small, though complicated in structure and branching into communities, scarcely ever of any great magnitude, humble creatures which have never played any great part in the world, have, nevertheless, been so persistent that, though specific and generic forms have been changed, the group may be said to be in the modern seas exactly what it was in those of the early Palæozoic, nor can it be affirmed to have originated in anything different, or to have produced anything. [Illustration: FIG. 41.—_a_, _Stenopora exilis_ (Dawson). _b_, _Chaetetes tumidus_ (Edwards and Haine). Carboniferous.]