ssic 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. 164) is here represented. Amongst the air-breathing
_Articulates_, we meet in the Oolitic rocks with the remains of
Spiders (_Arachnida_), Centipedes (_Myriapoda_), and numerous
true Insects (_Insecta_). In connection with the last-mentioned
of these groups, it is of interest to note the occurrence of
the oldest known fossil Butterfly--the _Paloeontina Oolitica_
of the Stonesfield slate--the relationships of which appear to
be with some of the living Butterflies of Tropical America.

[Illustration: Fig. 164.--_Eryon arctiformis_, a "Long-tailed
Decapod," from the Middle Oolites (Solenhofen Slate).]

Coming to the _Mollusca_, the _Polyzoans_, numerous and beautiful
as they are, must be at once dismissed; but the _Brachiopods_
deserve a moment's attention. The Jurassic Lamp-shells (fig.
165) do not fill by any means such a predominant place in the
marine fauna of the period, as in many Palaeozoic deposits, but
they are still individually numerous. The two ancient genera
_Leptoena_ (fig. 165, a) and _Spirifera_ (fig. 165, b), dating
the one from the Lower and the other from the Upper Silurian,
appear here for the last t