er Silurian. The
huge _Paradoxides_ of the Cambrian have now disappeared, and with
them almost all the principal and characteristic "primordial"
genera, save _Olenus_ and _Agnostus_. In their 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 a