JURASSIC PERIOD. 537 



LIFE OF THE PERIOD. The vegetation of the Jurassic 

 period is characterised by the abundance of Ferns, Cycads, 

 and Conifers the Cycadacece attaining here their maximum of 

 development. 



The Protozoa are represented by numerous Foraminifera 

 and by Sponges. Of the former the genera Involuting Nodo- 

 saria, Cristellaria, Dentalina, and Frondicularia may be men- 

 tioned as amongst the most important. Of the latter the chief 

 forms belong to Cribrospongia, Actinospongia, Porospongia, 

 Goniospongia, Perispongia, Chenendopora, and Scyphia. 



The Ccelenterates are represented in the Jurassic period by 

 numerous corals, which are exceedingly abundant in some of 

 the limestones of the series (such as the Coral-rag and the 

 Great Oolite). The number of Oolitic genera of Corals is 

 very large, but the commonest and most characteristic forms 

 belong to Thamnastrtza, Isastrcca, Prionastr&a, Anabaaa, Mont- 

 livaltia, T/iecosmilia, Eunomia, Protoseris, Comoseris, Den- 

 drarea, Dactylarea, Loboccenia, Aplophyllia, Trochocyathus, and 

 Stylina. 



The Echinoderms are very largely represented all through 

 the Jurassic Series. The Crinoids are represented both by 

 stalked forms (Pentacrinus, Extracrinus, Apiocrinus, &c.) and 

 by free forms (Saccosomd). Echinoids are extremely abundant 

 in many parts of the series, the commonest generic types being 

 HetnictdariS) Diadema, Pseudodiadema, Nucleolites, Dysaster 

 (Colly rites), Acrosalenia, and Cidaris. True Star-fishes ( Uraster, 

 Tropidaster, Plumaster, Solaster, and Astropecteri) are not un- 

 known, and Ophiuroids (Ophioderma, Ophiolepis, Acroura, &c.) 

 are far from uncommon. 



As regards the' Arthropods, Crustaceans are abundantly 

 found in certain beds (especially in the Solenhofen Slates). 

 The orders which are most largely represented are the Decapoda 

 (with many forms, both Macrurous and Brachyurous), the Cir- 

 ripedia, and the Ostracoda. Besides Crustaceans, the Oolitic 

 rocks have yielded numerous Insects, belong to the orders 

 Coleoptera, Neuroptera, Orthoptera, Hemiptera, Diptera, and 

 Hymenoptera. True Spiders have also been detected. 



Coming to the Mollusca, Brachiopods are still abundant, 

 though they do not fill such a predominant place in the marine 

 fauna as in many Palaeozoic deposits. The Palaeozoic genera 

 Leptcena and Spirifera appear here (in the Lias) for the last 

 time ; and most of the Jurassic forms belong to the modern 

 genera Terebratula and Rhynchonella. Bivalves are very abun- 

 dant, and approximate in many respects to existing forms. 

 The sub-genera Gryphaa and Exogyra amongst the Oysters, 



