PALEOZOIC TIME — CARBONIC. 727 



Corals also with few exceptions ; Nautiloids lose nearly all their Orthoceras- 

 like forms; while the coiled Nautilus-like species culminate in the Carbonif- 

 erous, and have few species and genera afterward. So the Insects had 

 Paleozoic features which were dropped at the same time, and one division 

 passed its time of culmination. The Placoderm, Dipnoan, and Ganoid Fishes, 

 which were eminently Paleozoic types, culminated in the Devonian and Car- 

 bonic eras, and only inferior Dipnoans and Ganoids existed later. Cryptog- 

 amous Plants culminated in the Carboniferous era, and only the Calamites 

 and some related genera, and a few genera of Ferns survived into the 

 Mesozoic. 



Should discovery open to view earlier species than those now known in 

 the Cambrian, they would be only earlier representatives of Paleozoic types, 

 or their precursor embryonic kinds. And if some of these latter existed in 

 preceding Archaean time, this fact would be parallel with the appearance of 

 many Mesozoic types in the course of Paleozoic time. 



The disappearance of species at the close of Paleozoic time was not due 

 chiefly to physical catastrophe, for the Trilobites had dwindled greatly by 

 the close of the Devonian ; and similar expansions to culmination in many 

 other tribes, with subsequently a commencing decline, have been mentioned 

 in the preceding pages, both among plants and animals. 



How far such culminations were a consequence primarily of laws of 

 growth it is not possible to say. There is no doubt as to their connection 

 with physical changes in progress. One of these physical changes was the 

 slow removal of carbonic acid from the atmosphere. The making of shells, 

 corals, and Crinoid skeletons, and thereby the making of limestones, was, 

 through Paleozoic time, dependent mainly on carbon abstracted from the 

 carbonic acid of the air and waters ; and vegetation, so far as its products 

 became stored in the rocks, in the form of coal, oil, gas, and other carbo- 

 naceous products, involved a further abstraction, as explained on page 485. 

 The purification of the air which was thus carried on was the means of fitting 

 it for Spiders, Insects, and other terrestrial life, and afterwards for Am- 

 phibians, and finally for Reptiles. Change in animal as well as vegetable 

 types must have been involved in this using up of the deleterious carbonic 

 acid. But the extent of its influence can only be conjectured. An examina- 

 tion into the amount of carbonic acid which air can contain Avithout being 

 injurious to different kinds of Insects, and to Amphibians, Reptiles, and 

 other species, would have much geological interest. Decline in the tempera- 

 ture of the sea and air through Paleozoic time also had its influence. But it 

 is not safe at present to attribute special facts to this cause. 



SECTION OF THE PALEOZOIC ROCKS OP PENNSYLVANIA. 



The following section of the Paleozoic rocks of Pennsylvania, published by H. D. 

 Rogers, after the fib:st survey of the state, is here added because of its geological and 

 historical value. 



