238 HISTORICAL PALAEONTOLOGY. 



the majority belong to the important reef -building family of 

 the "Star-corals" (Astrccida) . The form here figured (Thecos- 

 milia annularis, fig. 161) is one of the characteristic species 

 of the Coral-rag. 



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 



Fig. 161. Tkecosmilia annularis. Coral-rag, England. 



or " column. " One of the most characteristic Jurassic genera 

 of these " stalked " Crinoids (though not exclusively confined 

 io 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 Pentacrin- 

 ite" (Pentacrinus caput-medusa). 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 



