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.)]
The Lamp-shells or _Brachiopods_ are so numerous, and present
such varied types, both in this and the succeeding period of
the Upper Silurian, that the name of "Age of Brachiopods" has
with justice been applied to the Silurian period as a whole. It
would be impossible here to enter into details as to the many
different forms of Brachiopods which present themselves in the
Lower Silurian deposits; but we may select the three genera _Orthis,
Strophomena_, and _Leptoena_ for illustration, as being specially
characteristic of this period, though not exclusively confined to it.
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