384 



PALEOZOIC TIME. 



and these made up a very large part of the animal life of the Lower 

 Silurian. 



6. Harmony in the life of an era. — The forests of the Devonian 

 and Carboniferous were made up of Acrogens, or the highest of Cryp 

 togams, and Conifers, the lowest of Phenogams ; and among the for- 

 mer there were the pine-like Lepidodendrids and Sigillarids, having 

 the foliage of the Conifers, and somewhat also of their form of fructi 

 fication. In the Silurian, when the bivalved Mollusks were the most 

 abundant of species, Ostracoids, or bivalve Crustaceans, were also ex- 

 ceedingly common. 



7. Exterminations. — At the close of each period of the Paleozoic 

 ages, there was an extermination of a large number of living species. 

 Again, as each epoch terminated, there was an extermination of life, 

 but in most cases less general. With the transitions between strata of 

 different kinds, in the course of an epoch, there were usually some ex- 

 terminations ; and, even in the passage from layer to layer, there is 

 often evidence of the extinction of some species. In a corresponding 

 manner, there were often one or more new species with each new kind 

 of layer, and generally several with each change in the strata ; while 

 many appeared with the opening of an epoch, and a whole fauna, 

 nearly, with the commencement of a period. Hence, the introduction 

 and extinction of species were going on through the whole course of 

 the history, instead of being confined to particular points of time ; but, 

 at the close of long periods and epochs, there were more general ex- 

 terminations. As the rocks from which the facts come are Continental 

 rocks, the conclusion with regard to the completeness of extermina- 

 tions cannot be regarded as applying necessarily to the life of the 

 deeper parts of the ocean. 



8. Extinction of whole tribes, families, or genera of species. — Among 

 the tribes of land-plants of the Carboniferous age that became extinct 

 at its close, there are those of the Sigillarids and Lepidodendrids. 



The races of animals that were most prominent in giving a special 

 character to the Paleozoic fauna were the following : — 



Among Radiates, Crinoids and Cyathophylloid Corals ; among Mol- 

 lusks, Brachiopods and Orthocerata ; among Articulates, Trilobites ; 

 among Vertebrates, the vertebrate-tailed Ganoid fishes. Of these, the 

 group of Trilobites became extinct with the close of the Paleozoic, 

 and the vertebrate-tailed Ganoids very nearly so ; and Cyathophylloid 

 Corals, Crinoids, Brachiopods, and Orthocerata lost their preeminence 

 in numbers of species and individuals, in their respective sub-kingdoms. 



The following are a few other examples of the last appearance 

 among fossils of prominent Paleozoic groups : — 



Graptolites, which culminated in the Lower Silurian, became rare 

 before the close of the Upper Silurian, and ended with the Carbonif- 



