140 HISTORICAL GEOLOGY. 



Radiates began with the commencement of the Silurian, and have 

 continued till now, rather increasing throughout the ages. 



Mollusks had their beginning at the same time, and continued in- 

 creasing to the age of Reptiles : they then passed their maximum (as 

 indicated in the figure). 



Articulates commenced in the Silurian (as Crustaceans and Worms), 

 and continued expanding in numbers and grade to the present time. 



Fishes began near the close of the Silurian, were very abundant in 

 the Devonian, and continued on, becoming increasingly diversified to 

 the last, with some rise in grade. 



Reptiles began in the Carboniferous, and reached their maximum 

 in the Reptilian age. 



Mammals began in the Reptilian age, and were the highest race of 

 the Mammalian age. 



Sea-weeds (or Algae) were the earliest plants of the globe, probably 

 preceding animal life. Acrogens and Conifers began in the Upper 

 Silurian. The Acrogens bad their greatest expansion in the age of 

 Coal-plants, in which they occurred with Conifers. Cycads began in 

 the Carboniferous, and had their greatest expansion in the Reptilian 

 age. Dicotyledons began in the closing period of the Reptilian age, 

 and expanded, along with Palms, through the age of Mammals. 



The Silurian, Devonian, and Carboniferous ages naturally stand 

 somewhat apart from the following ones, in the peculiar ancient forms 

 of the great portion of their living tribes ; and to the whole collectively 

 the term Paleozoic era is appropriately applied, — the word "paleo- 

 zoic" being from the Greek 7raAato;, ancient, and £w7/. The follow- 

 ing age, or age of Reptiles, is correspondingly termed the Mesozoic, 

 from /xeo-os, middle, and {wt/, it being the mediaeval era in geological 

 history. The Mammalian age is termed the Cenozoic, from Katvo?, 

 recent, and t,wrj. (The words Eocene, Miocene, etc., subdivisions of the 

 age, are in part from the same root.) 



The subdivisions of geological time are, then, — 

 I. Archaean Time, including an Azoic and an Eozoic era though 

 not yet distinguished in the rocks. 



1. Azoic Age. 



2. Eozoic Age. 



II. Paleozoic Time. 



1. The Age of Invertebrates, or Silurian. 



2. The Age of Fishes, or Devonian. 



3. The Age of Coal-plants, or Carboniferous. 



III. Mesozoic Time. 



The Age of Reptiles. 



