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There are thus no undoubted fossil animals until the Cam- 

 brian period, where many existing groups appear to start 

 suddenly into being. We lind Radiolaria, Sponges, Graptolites, 

 Polyzoa, Bracliiopoda, Edriasteroidea, Carpoidea, Asteroidea, 

 Cha3topoda (worm-tubes), Phyllocarida, Ostracoda, Trilobites, the 

 generalised Insects known as Pala-odictyoptera, iso- and hetero- 

 myarian Pelecypoda, Gastropods (Prosobranchs and Pteropods), 

 and tetrabranchiate Cephalopods (Orthoceras, &c.) — all, it will be 

 noticed, marine forms, with the exception of Insects. 



Proceeding a stage onwards we find in the Silurian period, 

 in addition to the above groups, Foraminifera, Actinozoa (rugose 

 Corals), Ophiuroids, Echinoids, Crinoids, Girripedes, Scorpions, 

 Eurypterida, Amphineura, Scaphopoda, Elasmobranchii, and Ostra- 

 codermi. 



Thus, in the two earliest fossilifcrous 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 l^hyla, 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, Theromorpha, Sauropterygia, and Rhynchocephalia. This 

 period is also remarkable for the occurrence of Ceratodus, the 

 oldest still existing; o-enus of Vertebrates. 



Thus by the end of the Pala-ozoic 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 



n 11 2 



