place we have a great number of new forms—­some of them, like the great Asaphus tyrannus of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals.  Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist.  A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the principal Lower Silurian genera—­Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illoenus, AEglina, Harpes, Remopleurides, Phacops, Acidaspis, and Homalonotus, a few of them passing upwards under new forms into the Upper Silurian.

Coming next to the Mollusca, we find the group of the Sea-mosses and Sea-mats (Polyzoa) represented now by quite a number of forms.  Amongst these are examples of the true Lace-corals (Retepora and Fenestella), with their netted fan-like or funnel-shaped fronds; and along with these are numerous delicate encrusting forms, which grew parasitically attached to shells and corals (Hippothoa, Alecto, &c.); but perhaps the most characteristic forms belong to the genus Ptilodictya (figs. 48 and 49).  In this group the frond is flattened, with thin striated edges, sometimes sword-like or scimitar-shaped, but often more or less branched; and it consists of two layers of cells, separated by a delicate membrane, and opening upon opposite sides.  Each of these little chambers or “cells” was originally tenanted by a minute animal, and the whole thus constituted a compound organism or colony.

[Illustration:  Fig. 48.—­Ptilodictya falciformis. a, Small specimen of the natural size; b, Cross-section, showing the shape of the frond; c, Portion of the surface, enlarged.  Trenton Limestone and Cincinnati Group, America. (Original.)]

[Illustration:  Fig. 49.—­A, Ptilodictya acuta; B, Ptilodictya Schafferi. a, Fragment, of the natural size; b, Portion, enlarged to show the cells.  Cincinnati Group of Ohio and Canada.  (Original.)]

[Illustration:  Fig. 50.—­Lower Silurian Brachiopods. a and a’, Orthis biforata, Llandeilo-Caradoc, Britain and America:  b, Orthis flabellulum, Caradoc, Britain:  c, Orthis subquadrata, Cincinnati Group, America; c’, Interior of the dorsal valve of the same:  d, Strophomena deltoidea, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and Salter.)]