KETROSPECT OF PALEOZOIC AGE. 



The curious and interesting structure of the eyes of the trilobite 

 have been already noticed (62). 



298. The most striking and imposing feature of the fourth or 

 Carboniferous period of the Palaeozoic age, consists in the numerous 

 accumulations of vegetable remains which are presented in the 

 coal deposits in all parts of the world. As the science of Com- 

 parative Anatomy has enabled the geologist to reproduce the forms 

 and determine the habits and functions of the animal tribes 

 which peopled the earth at these remote periods in like manner 

 the principles of Botanical Science have enabled him, basing his 

 conclusion upon the visible structure of vegetable remains found 

 in the coal beds, to reproduce, as it were, the vast forests of 

 palms and arborescent ferns, the groves of conifers, and all the 

 exuberant vegetation which flourished during the second, third, 

 and fourth Pakeozoic periods. 



299. The layers of pure coal consist altogether of carbonised 

 vegetables ; and when it is considered that these strata are some- 

 times sixty feet in thickness, it seems difficult to explain how an 

 accumulation of wood, plants, and foliage could ever be produced 

 in such enormous quantity. Though the vegetable remains are 

 always in a carbonised state, the leaves often possess such tenacity 

 as to be separable from the stone. These and the seed-vessels 

 which are found in iron-stone, have in many cases undergone 

 metallic impregnation, which has in no degree impaired the 

 delicacy of their structure. The coal plants have been deter- 

 mined to the number of nearly a thousand species, two-thirds of 

 which are related to the ferns and the higher tribes of Crypto- 

 gamia, the remainder consisting of conifers and some flowering 

 monocotyledonous (having only one seed lobe), and dicotyledonous 

 (having two seed lobes) trees ; numerous species, however, are 

 still undescribed, and new forms are continually discovered. 



300. More than two hundred species of plants have been dis- 

 covered in the British coal-mines, but far greater numbers are 

 found in the Carboniferous deposits of Europe, America, Australia, 

 and even Greenland ; and it is worthy of note, that in the coal- 

 field on the shores of Lake Breton, fossil plants have been dis- 

 covered identical with those found in the coal-mines of Northum- 

 berland, though these deposits have been made in opposite sides 

 of the earth. 



301. The prominent character of the vegetable kingdom during 

 the Carboniferous period was the immense predominance of the 

 vascular and higher tribes of Cryptogamic plants, with which 

 were associated in a much less number, palms, conifers, cicadeee, 

 and other plants approaching to the character of Cactese and 

 Euphorbiacese. Plants analogous to the tribes of Ductulosse 



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