" Lower Silurian, Canada. (After
Billings.)]
In addition to the Rugose Corals, the Lower Silurian rocks contain
a number of curious compound corals, the tubes of which have
either no septa at all or merely rudimentary ones, but which
have the transverse partitions or "tabulae" very highly developed.
These are known as the _Tabulate Corals_; and recent researches
on some of their existing allies (such as _Heliopora_) have shown
that they are really allied to the modern Sea-pens, Organ-pipe
Corals, and Red Coral, rather than to the typical stony Corals.
Amongst the characteristic Rugose Corals of the Lower Silurian
may be mentioned species belonging to the genera _Columnaria,
Favistella, Streptelasma_, and _Zaphrentis_; whilst amongst the
"Tabulate" Corals, the principal forms belong to the genera
_Choetetes, Halysites_ (the Chain-coral), _Constellaria_, and
_Heliolites_. These groups of the Corals, however, attain a greater
development at a later period, and they will be noticed more
particularly hereafter.
[Illustration: Fig. 46.--Group of Cystideans. A, _Caryocrinus
ornatus_,[13] Upper Silurian, America; B, _Pleurocystites squamosus_,
showing two short "arms," Lower Silurian, Canada; C, _Pseudocrinus
bifasciatus_, Upper Silurian, England; D, _Lepadocrinus Gebhartii_,
Upper Silurian, America. (After Hall, Billings, and Salter.
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