s. The occurrence of this genus is of
interest, because it is the first known instance in the entire
geological series of the occurrence of an unquestionable Hydroid of
a modern type, though many of the existing forms of these animals
possess structures which are perfectly fitted for preservation in
the fossil condition. 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. From the many genera of Sea-urchins which occur in strata
of this age, it is difficult to select characteristic types;
but the genera _Galerites_ (fig. 191), _Discoidea_ (fig. 192),
_Micraster, Ananchytes, Diadema, Salenia_, and _Cidaris_, may
be mentioned as being all important Cretaceous groups.

Coming to the _Annulose Animals_ of the Cretaceous period, there
is little special to remark. The _Crustaceans_ belong for the
most part to the highly-organised groups of the Lobsters and the
Crabs (the Macrurous and Brachyurous Decapods); but there are
also numerous little _Ostracodes_, especially in the fresh-water
strata of the Wealden. It should further be noted that there
occurs here a great development of the singular _Crustaceous_
fam