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.)]