604 ZOOLOGY SECT. 



branchiate Gastropods, and dibranchiate Cephalopods (Belemnites). 

 The palaeozoic types of Tetrabranchs (Orthoceras, etc.) have nearly- 

 disappeared, and the Ammonites have become important. Among 

 Vertebrates are found Holostei, Chelonia, Ichthyopterygii, Croco- 

 dilia, and Dinosauria, the latter especially being a very prominent 

 group, as well as several Mammalia (Microlestes, Hypsiprym- 

 nopsis, etc.) of uncertain affinities. 



In the Jurassic period the two highest orders of Insects, 

 Hymenoptera and Lepidoptera, are known for the first time, as 

 well as the reptilian Ornithosauria, and the earliest known Bird 

 (Archceopteryx). There are also several small Mammals (Pla- 

 giaulax, Amphithermm, Phascolotherium, etc.) belonging either to 

 the Prototheria or to the Metatheria, but occurring in Europe 

 and North America, where there are at present with the excep- 

 tion of the Opossums no representatives of either order. This 

 seem to indicate that Mammals originated in the northern 

 hemisphere and spread southwards. 



In the Cretaceous period the Crab the most specialized of 

 the higher Crustacea and the Teleostei the most specialized of 

 Fishes make their appearance. Of the last-named group, several 

 Cretaceous genera survive and flourish to the present day, e.g., 

 Clupea (Herring), Esox (Pike), Osmerus (Smelt), and Beryx. Ophidia 

 are known for the first time, and Pythonomorpha, Dinosaurs, and 

 Ornithosaurs are important. Mammals are practically unknown, 

 but among Birds the Odontolcae and the Ichthyoraithes are 

 characteristic. By the end of the period five entire groups of 

 Reptiles the Sauropterygia, Ichthyopterygia, Pythonomorpha, 

 Dinosauria, and Ornithosauria have become extinct, none of them 

 being known to extend into Tertiary times. 



Except in California and Patagonia there is a well-marked break 

 between the Cretaceous and the Eocene periods, the fauna of the 

 latter having a comparatively modern character. The Pelecypods 

 and Gastropods belong to existing families and even to existing 

 genera, and Belemnites have almost, and Ammonites quite, dis- 

 appeared. The Fishes all belong to existing types ; Stegocephali 

 have given place to Urodela and Anura, and none of the Reptiles 

 belong to extinct orders. Among Birds, the Penguins, Gulls, Rails, 

 Owls, Picarians (Kingfishers, etc.), and Passeres have appeared, as 

 well as the extinct orders Stereornithes and Gastornithes, and the 

 goose-like Odontopteryx. 



But the most noticeable feature of the period is the rise and 

 differentiation of the Mammalia. Among existing orders the 

 Marsupialia (Opossums), Cetacea (Ze^t,glodon) ) Sirenia (Eoiheriuni), 

 Ungulata,Carnivora, Insectivora,Chiroptera, and Primates (Lemurs) 

 appear for the first time, as well as the extinct orders Creodonta, 

 Condylarthra, Amblypoda and Tillodontia, together with the 

 Dinocerata, none of which extend beyond the Eocene period. 



