The Echinoderms are very numerous and abundant fossils in the Jurassic series, and are represented by Sea-lilies, Sea-urchins, Star-fishes, and Brittle-stars.  The Crinoids are still common, and some of the limestones of the series are largely composed of the debris of these organisms.  Most of the Jurassic forms resemble those with which we are already familiar, in having the body permanently attached to some foreign object by means of a longer or shorter jointed stalk or “column.”  One of the most characteristic Jurassic genera of these “stalked” Crinoids (though not exclusively confined to this period) is Pentacrinus (fig. 162).  In this genus, the column is five-sided, with whorls of “side-arms;” and the arms are long, slender, and branched.  The genus is represented at the present day by the beautiful “Medusa-head Pentacrinite” (Pentacrinus caput-medusoe).  Another characteristic Oolitic genus is Apiocrinus, comprising the so-called “Pear Encrinites.”  In this group the column is long and rounded, with a dilated base, and having its uppermost joints expanded so as to form, with the cup itself, a pear-shaped mass, from the summit of which spring the comparatively short arms.  Besides the “stalked” Crinoids, the Jurassic rocks have yielded the remains of the higher group of the “free” Crinoids, such as Saccosoma.  These forms resemble the existing “Feather-stars” (Comatula) in being attached when young to some foreign body by means of a jointed stem, from which they detach themselves when fully grown to lead an independent existence.  In this later stage of their life, therefore, they closely resemble the Brittle-stars in appearance.  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