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 "tabulæ" 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.)] [Footnote 13: The genus _Caryocrinus_ is sometimes regarded as properly belonging to the _Crinoids_, but there seem to be good reasons for rather considering it as an abnormal form of _Cystidean_.]