620 ZOOLOGY SECT. 



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

 tion of the Opossums no representatives of either order. This 

 seems to indicate that the lower Mammals originated in the 

 northern hemisphere and spread southwards. 



In the Cretaceous period the Crabs the most specialised of 

 the higher Crustacea and the Teleosts the most specialised 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 Ichthyornithes 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 ; Stegocephala 

 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, &c.) 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 (Zeuglodon), Sirenia (Prorastomus, 

 Eosiren), 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. In the lower Eocene none of the Mammals belong to exist- 

 ing genera, but in the upper Eocene are found Didelphys (Opossum), 

 Rhinoceros, Viverra (Civet), Mustela (Weasel), and possibly Canis. 

 The period is also remarkable for the number of annectent or 

 linking forms. There are, for instance, species connecting Dogs 

 with Bears and with Civets, Civets with Hyaenas, Hyaenas with 

 Cats, Pigs with Pecora, Deer with Chevrotains, Tapirs with Rhino- 

 ceroses and with Horses, and so on. It is perfectly clear that the 

 orders, sub-orders, and families of Mammalia, as we now under- 

 stand them, were, during the Eocene period, becoming gradually 

 differentiated from common ancestral forms. 



In the Miocene period the Proboscidea (Elephant and Mastodon) 

 make their appearance, as well as Gibbon-like Anthropoidea 



