C, Fragment of the
same viewed in front, showing the mouths of the cellules. D,
Cross-section of the same. From the Wenlock Group (Coniston Flags
of the North of England). (Original.)]
The _Corals_ are very numerously represented in the Upper Silurian
rocks some of the limestones (such as the Wenlock Limestone)
being often largely composed of the skeletons of these animals.
Almost all the known forms of this period belong to the two great
divisions of the Rugose and Tabulate corals, the former being
represented by species of _Zaphrentis, Omphyma, Cystiphyllum,
Strombodes, Acervularia, Cyathophyllum_, &c.; whilst the latter
belong principally to the genera _Favosites, Choetetes, Halysites,
Syringopora, Heliolites_, and _Plasmopora_. Amongst the _Rugosa_, the
first appearance of the great and important genus _Cyathophyllum_,
so characteristic of the Palaeozoic period, is to be noted; and
amongst the _Tabulata_ we have similarly the first appearance,
in force at any rate, of the widely-spread genus _Favosites_--the
"Honeycomb-corals." The "Chain-corals" (_Halysites_), figured
below (fig. 59), are also very common examples of the Tabulate
corals during this period, though they occur likewise in the
Lower Silurian.
[Illustration: Fig. 59.--a, _Halysites catenularia_, small variety,
of the natural size; b, Fragment of a large variety of the same,
of the natural size; c, Fragment of limestone with the tubes
of _Halysites agglomerata_, of the natural size; d, Vertical
section of two tubes of the same, showing the tabulae, enlarged.
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