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_
family of the Barnacles (_Lepadidoe_), whilst the allied family
of the equally singular Acorn-shells (_Balanidoe_) is feebly
represented as well.
[Illustration: Fig. 191.--_Galerites albogalerus_, viewed from
below, from the side, and from above. White Chalk.]
[Illustration: Fig. 192.--_Discoidea cylindrica_; under, side,
and upper aspect. Upper Greensand.]
Passing on to the _Mollusca_, the class of the Sea-mats and
Sea-mosses (_Polyzoa_) is immensely developed in the Cretaceous
period, nearly two hundred species being known to occur in the
Chalk. Most of the Cretaceous forms belong to the family of the
_Escharidoe_, the genera _Eschara_ and _Escharina_ (fig.
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