True Star-fishes (_Asteroids_) and Brittle-stars
(_Ophiuroids_) are abundant in the Jurassic rocks, and the
Sea-urchins (_Echinoids_) are so numerous and so well preserved
as to constitute quite a marked feature of some beds of the series.
All the Oolitic urchins agree with the modern _Echinoids_ in
having the shell composed of no more than twenty rows of plates.
Many different genera are known, and a characteristic species
of the Middle Oolites (_Hemicidaris crenularis_, fig. 163) is
here figured.
[Illustration: Fig. 163.--_Hemicidaris crenularis_, showing the
great tubercles on which the spines were supported. Middle Oolites.]
Passing over the _Annelides_, which, though not uncommon, are
of little special interest, we come to the _Articulates_, which
also require little notice. Amongst the _Crustaceans_, whilst
the little Water-fleas (_Ostracoda_) are still abundant, the
most marked feature is the predominance which is now assumed by
the _Decapods_--the highest of the known groups of the class.
True Crabs (_Brachyura_) are by no means unknown; but the principal
Oolitic Decapods belonged to the "Long-tailed" group (_Macrura_),
of which the existing Lobsters, Prawns, and Shrimps are members.
The fine-grained lithographic slates of Solenhofen are especially
famous as a depot for the remains of these Crustaceans, and a
characteristic species from this locality (_Eryon arctiformis_,
fig.
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