TIIK JUBAS8K SVSTKM 385 



Another special feature of the fauna is the great abundance and 

 vaiii'ty of tin- Ammonoid Cephalopoda. A few genera such aa 

 Phylloceras appeared in Triaasic times, but a large number occur in 

 io rocks, and many are confined to the period. The genus 

 Xn a til us, which also dates from the Triassic period, is the only 

 representative of the Nautiloidea. Belemnites, a genus of 

 Dibrunchiate Cephalopods, appears in the Lias and reaches its 

 maximum development in Jurassic times. 



Gastropods are numerous, especially Nerincea, Pleurotomaria, 

 Ci mix, Pseudomelania, Natica, and Amberlya. 



Among Lamellibranchia the Ostreidae, Pectinida j , Limidae, and 

 Pholadomyidae become abundant, and the genus Gryphcea is very 

 characteristic of the period ; Trigonia also is abundant 



Among Echinoderrns the Euechinoid Group makes its appearance 

 in England, and is very abundant both in species and individuals. 

 The first irregular Echinoid is found in the Upper Lias. Some 

 representatives of the Asteroidea, Ophiuroidea, and Crinoidea also 

 occur. Corals also abound in most of the limestones. 



Plants occur at several horizons, and Cycad-like fronds, stems, 

 and " flowers " are so abundant that botanists have called the 

 Jurassic period the "age of Cycads." The chief genera are 

 Zamites, Otozamites, Nilssonia, Fterophylhim, and Williamsonia. 

 Ferns continue to be common, and conifers were represented by 

 genera allied to the modern Araucaria, Cupressus (cypress), and 

 Taxus (yew). 



The following are some of the genera which are not known in 

 Britain or France before Lower Jurassic time, though some occur 

 in the Alpine Trias. Those preceded by an asterisk did not survive 

 the Jurassic period. 



Anthozoa. Astrococnia, *Heterastrea, Isastrea, Montlivaltia, 

 *Stylastrea, Thamnastrea, Thecosmilia, Trochocyathus. 



Echinoderma. *Pentacrimis, Isocrinus, Ophiodenna, Ophio- 

 lepis, Acrosalenia, Cidaris, Rhabdocidaris, Hemipedina, Pseudo- 

 diadema. 



Crustacea. *JEger, *Eryma, Eryon, Glyphaea, Penanis, 

 *Scapheus. 



Brachiopoda. Waldheimia, Thecidium, Terebratella, Cado- 

 mella. 



Lamellibranchia. AstartejCaixliniajCarditajCardiunijProto- 

 caixlia, *Ceromya, Grammatodon, Cucullaea, Trapezium, *Gresslya, 

 Gryphtea, Velopecten, *Hippopodium, Homomya, Inoceramus, 

 Myoconcha, Opis, Ostrea, Pliaitula, Perna, Pinna, Pholadoniya, 

 Goniomya, Pleuromya, *Tancredia, Thracia, Trigonia, Unicardium. 



Q-astropoda. Actaeonina, Amberleya, Cerithiuni, *Ceritella, 



2C 



