116, a), sometimes simple, sometimes forming great masses of star-like corallites; and _Choetetes_, with its branched stems, and its minute, "tabulate" tubes (fig. 116, f). The above, together with other and hardly less characteristic forms, combine to constitute a coral-fauna which is not only in itself perfectly distinctive, but which is of especial interest, from the fact that almost all the varied types of which it is composed disappeared utterly before the close of the Carboniferous period. In the first marine sediments of a calcareous nature which succeeded to the Coal-measures (the magnesian limestones of the Permian), the great group of the _Rugose corals_, which flourished so largely throughout the Silurian, Devonian, and Carboniferous periods, is found to have all but disappeared, and it is never again represented save sporadically and by isolated forms. [Footnote 19: A singular fossil has been described by Professor Martin Duncan and Mr Jenkins from the Carboniferous rocks under the name of _Paloeocoryne_, and has been referred to the Hydroid Zoophytes (_Corynida_). Doubt, however, has been thrown by other observers on the correctness of this reference.] [Illustration: Fig. 117.--_Platycrinus tricontadactylus_, Lower Carboniferous. The left-hand figure shows the calyx, arms, and upper part of the stem; and the figure next this shows the surface of one of the joints of the column. The right-hand figure shows the proboscis. (After M'Coy.)] [Illustration: Fig. 118.--A, _Pentremites pyriformis_, side-view of the body ("calyx"); B, The same viewed from below, showing the arrangement of the plates; C, Body of _Pentremites conoideus_, viewed from above. Carboniferous.]