912 PRINCIPLES OF STRATIGRAPHY 



organic succession which parallels the stratigraphic succession in 

 the development of the earth's crust. It has been found that cer- 

 tain groups of organisms did not extend beyond a certain time 

 period in the earth's history, and so their occurrence in the strata 

 indicates the upper age limit of these strata. Thus, trilobites are 

 known only from Palaeozoic formations — hence the finding of one 

 of these extinct organisms stamps the strata in which it occurs 

 as of Palaeozoic age. More restricted occurrences of special forms 

 have made it possible to recognize the geological levels with greater 

 precision, thus the trilobite Paradoxides characterizes only the 

 Middle Cambric rocks of the earth's crust in certain regions, while 

 the trilobite Holmia characterizes the lower Cambric strata in the 

 same regions. Again, the Hydrozoan Dictyoncuia fJabellifonne is 

 of world-wide distribution in the basal Ordovicic strata. Organisms 

 used in this manner become indices of geological horizons and, since 

 all but those of the present geological period occur only in fossil 

 form, the term "index fossil" is properly applied to them. This 

 subject will be considered further in a subsequent chapter. 



CLASSIFICATION OF PLANTS AND ANIMALS. 



The classification of plants and animals is a process of assort- 

 ment into natural groups, or groups of related types, and the ar- 

 rangement of these groups in a natural order, according to their 

 genetic relationship. Groups of various denominations are rec- 

 ognized, the smallest of those in general use being the spe- 

 cies. Smaller groups, known as varieties or as mutations, are, 

 however, included within the species. Species are grouped into 

 genera, each genus comprising one or more species. The generic 

 and specific name of an organism are always used together in 

 speaking of any particular species, since the same specific name 

 may be used for a species of another genus. Thus, the fossil brach- 

 iopod genus Productus includes among its many species Productus 

 muricatus. Likewise the fossil pelecypod genus Actinopteria has 

 among its species Actinopteria muricata, while another brachiopod 

 genus, Strophalosia, also has a species Strophalosia muricata. 

 These three species, though they have the same specific name, are 

 not at all related to each other, belonging to distinct genera, one 

 of which belongs to a different division or phylum of the animal 

 kingdom from that to which the other two belong. 



Naming of genera and species. (i8.) The generic name is 

 always a noun, and is commonly derived from the Greek, though 



