The second great group of Coelenterate animals (Actinozoa) is represented in the Lower Silurian rocks by numerous Corals.  These, for obvious reasons, are much more abundant in regions where the Lower Silurian series is largely calcareous (as in North America) than in districts like Wales, where limestones are very feebly developed.  The Lower Silurian Corals, though the first of their class, and presenting certain peculiarities, may be regarded as essentially similar in nature to existing Corals.  These, as is well known, are the calcareous skeletons of animals—­the so-called “Coral-Zoophytes”—­closely allied to the common Sea-anemones in structure and habit.  A simple coral (fig. 43) consists of a calcareous cup embedded in the soft tissues of the flower-like polype, and having at its summit a more or less deep depression (the “calice”) in which the digestive organs are contained.  The space within the coral is divided into compartments by numerous vertical calcareous plates (the “septa"), which spring from the inside of the wall of the cup, and of which some generally reach the centre. Compound corals, again (fig. 44), consist of a greater or less number of structures similar in structure to the above, but united together in different ways into a common mass. 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.”  Lower Silurian, Canada. (After Billings.)]