CHARACTERISTIC GENERA AND SPECIES. 



It happens sometimes that particular species prevail through 

 certain groups of strata, superposed in regular order, but totally 

 disappear from all subsequent and antecedent. Such species 

 are accordingly characteristic, not indeed of particular strata, 

 but of those limited groups of strata in which they prevail, 

 and the inference is, that they had continued to exist upon the 

 earth during the period corresponding with the deposition of the 

 groups, but did not exist there before or after. 



60. The species which thus prevailed upon the earth during 

 geological periods more or less limited, and which ceased to exist 

 before the period marked by the presence of the human race, have 

 accordingly supplied to the geologist tests for the identification of 

 strata much more determinate than any which depend on their 

 mineral constituents. When the presence of a particular species 

 is strictly limited to particular strata, it becomes an unerring test 

 of the presence of that stratum wherever this species is found. If 

 it has prevailed through groups of strata, it is a test, though not of 

 particular strata, still of the group to which its presence is limited. 



61. Numerous examples of these characteristic species may be 

 mentioned. A certain, family of Crustacea, called TEILOBITES, 

 are almost exclusively limited to the Silurian period, appearing 

 rarely in the lower bed of the carboniferous limestone, and never 

 above them. 



62. The Trilobites consisted of an oblong body, divided trans- 

 versely into three parts, and also longitudinally into the same 



Fig. 0. (Trilobite) Ogygia Guettardi. 



number of lobes. The comparison of the forms of these animals 

 with those of existing Crustacea, renders it probable that they 

 dwelt in the depths of the sea, far from coasts, floating on their 

 back, and never resting, inasmuch as their feet could not retain 

 them stationary, and movement was necessary for their respira- 

 tion. From a peculiarity of the mouth, it is inferred that they 

 were carnivorous, preying probably on naked Mollusca or 



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