_Simple_ corals, therefore, are the skeletons
of _single_ and independent polypes; whilst _compound_ corals
are the skeletons of assemblages or colonies of similar polypes,
living united with one another another as an organic community.
[Illustration: Fig. 43.--_Zaphrentis Stokesi_, a simple "cup-coral,"
Upper Silurian, Canada. (After Billings.)]
[Illustration: Fig. 44.--Upper surface of a mass of _Strombodes
pentagonus_. Upper Silurian, Canada. (After Billings.)]
In the general details of their structure, the Lower Silurian
Corals do not differ from the ordinary Corals of the present
day. The latter, however, have the vertical calcareous plates
of the coral ("septa") arranged in multiples of six or five;
whereas the former have these structures arranged in multiples
of four, and often showing a cross-like disposition. For this
reason, the common Lower Silurian Corals are separated to form
a distinct group under the name of _Rugose_ Corals or _Rugosa_.
They are further distinguished by the fact that the cavity of
the coral ("visceral chamber") is usually subdivided by more
or less numerous horizontal calcareous plates or partitions,
which divide the coral into so many tiers or storeys, and which
are known as the "tabulae" (fig. 45).
[Illustration: Fig. 45.--_Columnaria alveolata_, a Rugose compound
coral, with imperfect septa, but having the corallites partitioned
off into storeys by "tabulae.
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