464 STRATIGRAPHICAL GEOLOGY 



B. LIFE OF THE PERIOD 



Flora. Comparatively few traces of the vegetation of the 

 period are found in the Cretaceous rocks of Britain except in the 

 Wealden Beds, in which, as in the Jurassic System, the pre- 

 dominating groups are ferns and cycads. A few conifers, horse- 

 tails (Equisetaceae), and a Chara also occur, but Dicotyledons are 

 unknown, although a few, the earliest of their class, have been found 

 in the Lower Cretaceous rocks of Spain and of North America. 



The flora of the Wealden presents a considerable resemblance 

 to that of the Jurassic strata, but as the early Cretaceous continent 

 was only an extension of the Upper Jurassic land, such a resemblance 

 should not cause any surprise. 



In the Upper Cretaceous of the Continent Dicotyledons form an 

 important part of the flora, and include a number of genera which 

 have living representatives, e.g. Acer (maple), Cinnamomum (cin- 

 namon), Ilex (holly), Quercus (oak), Platanus (plane), Hedera (ivy), 

 Populus (poplar), Myrica (myrtle), with Aralia, Magnolia, Euca- 

 lyptus, and the extinct genus Gredneria. Cycads are less numerous 

 than before ; subsequently they became rare in Europe, and at 

 the present day are chiefly found in the tropical and sub-tropical 

 parts of Asia, Africa, Australia, and Mexico. 



Fauna. The general aspect of the Cretaceous fauna resembles 

 that of the preceding Jurassic much more than that of the succeed- 

 ing Eocene. Throughout the Lower Cretaceous Series many genera 

 which were abundant in Jurassic times are still common, such as 

 Cucullcea, Trigonia, Gervillia, Perna, Pecten, Lima, Pleuromya, 

 and Pleurotomaria. The Ammonoid genera Holcostephanus and 

 Perisphinctes continue to be abundant, and the Belemnitidce were 

 nearly as common, but many new genera of Cephalopoda appear, 

 some in the lower and some in the upper division of the system. 

 The Echinoid genera Holectypus, Nucleolites, and Py gurus also 

 survive into the Lower Cretaceous, but many new genera make 

 their appearance (see list below). 



Passing to vertebrate animals, most of the Lower Cretaceous fish 

 belong to genera which existed in Jurassic times, but Edaphodon, 

 Lamna, and Protosphyrcena now appear, while in the Chalk many 

 Teleostean fish resembling modern genera are found, such as 

 Hoplopteryx, Osmeroides, Enchodus, and Portheus with the Elasmo- 

 branch Corax. Among reptiles the marine Ichthyosaurus and the 

 Dinosaurian genera Ornithopsis and Megalosaurus survive from 

 Jurassic time, but the Plesiosaurs are represented by Cimoliosaurus 

 and Polyptychodon. Fresh Dinosaurs make their appearance, e.g. 

 Iguanodon, Hypsilophodon, Hylceosaurus, Polacanthus, and Vecti- 



