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 _débris_ 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.]