The corals of the Cretaceous series are not
very numerous, and for the most part are referable to types such
as _Trochocyathus, Stephanophyllia, Parasmilia, Synhelia_ (fig.
190), &c., which belong to the same great group of corals as the
majority of existing forms. We have also a few "Tabulate Corals"
(_Polytremacis_), hardly, if at all, generically separable from very
ancient forms (_Heliolites_); and the Lower Greensand has yielded
the remains of the little _Holocystis elegans_, long believed to
be the last of the great Palaeozoic group of the _Rugosa_.
[Illustration: Fig. 190.--_Synhelia Sharpeana_. Chalk, England.]
As regards the _Echinoderms_, the group of the _Crinoids_ now
exhibits a marked decrease in the number and variety of its types.
The "stalked" forms are represented by _Pentacrinus_ and
_Bourgueticrinus_, and the free forms by Feather-stars like our
existing _Comatuloe_; whilst a link between the stalked and free
groups is constituted by the curious "Tortoise Encrinite
(_Marsupites_). By far the most abundant Cretaceous Echinoderms,
however, are Sea-urchins (_Echinoids_); though several Star-fishes
are known as well. The remains of Sea-urchins are so abundant
in various parts of the Cretaceous series, especially in the
White Chalk, and are often so beautifully preserved, that they
constitute one of the most marked features of the fauna of the
period.
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