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.)]

[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.]

Passing onto higher animals, we find that the class of the Echinodermata is represented by examples of the Star-fishes (Asteroidea), the Sea-lilies (Crinoidea), and the peculiar extinct group of the Cystideans (Cystoidea), with one or two of the Brittle-stars (Ophiuroidea)—­the Sea-urchins (Echinoidea) being still wanting.  The Crinoids, though in some places extremely numerous, have not the varied development that they possess in the Upper Silurian, in connection with which their structure will be more fully spoken of.  In the meanwhile, it is sufficient to note that many of the calcareous deposits of the Lower Silurian are strictly entitled to the name of “Crinoidal limestones,” being composed in great part of the detached joints, and plates, and broken stems, of these beautiful but fragile organisms (see fig. 12).  Allied to the Crinoids are the singular creatures which are known as Cystideans (fig. 46).  These are generally composed of a globular or ovate body (the “calyx"), supported upon a short stalk (the “column"), by which the organism was usually attached to some foreign body.  The body was enclosed by closely-fitting calcareous plates, accurately jointed together; and the stem was made up of numerous distinct pieces or joints, flexibly united to each other by membrane.  The chief distinction which strikes one in comparing