THE PEE-ADAMITE EAETH. 



containing its own peculiar organic deposits. In like manner, the 

 Triassic formation consists of two such stages, the Jurassic of ten, 

 the Cretaceous of seven, and, in fine, the Tertiary of five; so that the 

 whole fossiliferous portion of the earth's crust may be considered 

 as consisting of twenty-nine stages, each stage being a catacomb 

 in which the remains of the preceding creation are buried. 



240. The intervals 0f time during which each of these six for- 

 mations were deposited, we shall call geological ages. Thus, that 

 in which the Azoic formation was deposited we shall call the Azoic 

 aye,' that in which the Palaeozoic formation was deposited we shall 

 call the Paleozoic age, and so on. 



241. The lesser intervals during which the several stages com- 

 posing each geological formation were deposited, and during 

 which, as already stated, an animated world was created, lived,, 

 and was destroyed, we shall call geological periods. Thus the Palaeo- 

 zoic age consisted of five periods, denominated in their numerical 

 order, from the earliest or lowest to the latest or uppermost, the 

 first Palaeozoic period, the second Palceozoic period, and so on. In 

 the same manner, the Triassic age consisted of two periods, the 



Jirst and the second. The Jurassic of ten counted from the lowest, 

 or most ancient, to the highest or most modern, and the like of the 

 other formations. 



242. Since it will be necessary to make frequent reference to 

 the various forms of animal life, which from period to period pre- 

 vailed upon the earth ; and such references must occasionally 

 necessitate the use of certain technical zoological terms, it will be 

 convenient for those readers who are not already familiar with 

 the elements of zoology, to give a general outline of the classifi- 

 cation of animals which we shall adopt. 



Naturalists have classed all the various forms of animal life 

 into four primary divisions, denominated from their peculiar 

 structure: 1. VERTEBRATA; 2. ANNTTLATA ; 3. MOLLTJSCA; and 

 4. EADIATA.* 



243. Each of these primary divisions is resolved into a certain 

 number of CLASSES; each class is again resolved into a certain 

 number of ORDERS ; each order is resolved into a certain number 

 of GENERA ;t each genus is resolved into a certain number of 

 SPECIES, and each species consists of certain VARIETIES. 



Here the classification terminates, the varieties being composed 

 of individuals. Thus, if it be required to determine the zoological 

 character of any individual animal, it is first necessary to state 



* Much confusion prevails among the classifications adopted by natural- 

 ists ; we stall, however, generally adhere to that of Cuvier. 



f By an intermediate step, the orders are sometimes first grouped in 

 families, and the families subdivided into genera. 

 30 



