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Thus, in the two earliest fossiliferous systems are found repre- 

 sentatives of all the skeleton-forming phyla, i.e., of all but 

 Platyhelminthes, Nemathelminthes, and Trochelminthes. And, 

 as far as our present knowledge goes, there is no indication of 

 any connecting link between one phylum and another, the 

 primary divisions of the animal kingdom having been apparently 

 as well characterised at that enormously distant epoch as at 

 the present day. Obviously all the older or more generalised 

 animal types, which, if we reason from analogy, we must suppose 

 to have preceded the present well-marked phyla, have been 

 destroyed by metamorphic action or otherwise, without leaving 

 a trace of their existence. 



The Devonian period is remarkable for its abundant remains 

 of Fishes ; Crossopterygii, Chondrostei, and Dipnoi appear for 

 the first time, and all three groups of Ostracodermi are abundant. 

 Decapod Crustacea, of the macrurous or Shrimp type, also make 

 their appearance. In the Carboniferous period, notable for 

 its immense forest-flora, there is a great development of air- 

 breathing forms, such as Insects, Arachnids (Spiders), and 

 Myriapods, as well as Stegocephala, the earliest amphibious 

 Vertebrates. In the Permian rocks true air-breathing Verte- 

 brates first make their appearance in the form of the reptilian 

 orders Thermomorpha, Sauropterygia, and Rhynchocephalia. This 

 period is also remarkable for the occurrence of Ceratodus, the 

 oldest still existing genus of Vertebrates. 



Thus by the end of the Palaeozoic era, every important class of 

 animals capable of leaving fossil remains is represented, with the 

 exception of Mammalia and Birds. Moreover, the Trilobites, 

 the Eurypterida, the Pala?odictyoptera, and the Ostracodermi come 

 to an end during this era, no remains of them being known in 

 rocks of secondary age. 



In the succeeding Mesozoic era, the Triassic period intro- 

 duces existing orders of Insects Orthoptera, Neuroptera, and 

 Coleoptera, as well as Xiphosura, siphoniate Pelecypods, opistho- 

 branchiate Gastropods, and dibranchiate Cephalopods (Belemnites). 

 The Palaeozoic types of Tetrabranchs (Orthoceras, &c.) 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, &c.) 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 

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

 giaulax, AmpJiitherium, Phascolotherium, &c.) belonging either to 

 the Prototheria or to the Metatheria, but occurring in Europe 



