THE PALEONTOLOGIC RECORD 593 



such there may be in the present seas about 250,000 kinds, of which 

 about 25,000 have been named. Therefore on this basis we can say 

 that the student of Jurassic faunas knows 1 species in every 54 of 

 shelled animals that lived during this period. 



This admittedly great imperfection of the life record needs to be 

 further explained so that the reader will not arrive at the erroneous 

 conclusion that modern stratigraphy rests upon very insecure founda- 

 tions. The stratigrapher in determining the age of a given deposit, 

 and in the identification of it from place to place and from country to 

 country, and even across the great oceans, deals in his work not with 

 quantity of species, but with comparatively small numbers of con- 

 stantly recurring hard parts of certain species that are more often of 

 marine than of land origin. Many of these forms have but local value 

 but others have spread thousands of miles, and some of the long en- 

 during species range over the greater part of the earth. Some of the 

 best guide fossils in the Paleozoic are the brachiopods because they 

 are present in nearly all the strata of this era. The writer in 1897 

 listed 1,859 forms then known from these rocks of North America. 

 Of these about 28 per cent., or 537 species, had great geographic dis- 

 tribution. 117 species are found in the Eocky Mountain area, the 

 Mississippi valley and the Appalachian region, and of these 36 are also 

 known to occur in foreign countries. The number of species common 

 to North America and other continents, however, is 121. It is upon 

 faunal assemblages of this quantity and nature that the stratigrapher 

 relies most in deciphering the former extent of the continental seas. 



In the making of paleogeographic maps or in the determination of 

 geologic time, using fossils as the essential basis, we have guidance in 

 those of marine faunas, and the floras and faunas of the land and its 

 fresh waters. Of these widely differing realms or habitats we now 

 know that the fossils of the marine faunas are the more reliable not 

 only because there are so many more of them than of the land dwellers, 

 but more especially because their geologic succession is far more com- 

 plete. The conditions of preservation, that is, appropriate burial in 

 sediments, are always at hand in marine waters, but on the land en- 

 tombment occurs only exceptionally, whereas the life of fresh waters 

 is very meager and almost unchanging during geologic time. Then, 

 too, marine life is " less affected by meteorologic factors, and more 

 dependent upon conditions which affect the whole hydrosphere rather 

 than small areas of it. The struggle for life is less intense, the food 

 supply generally more adequate, enemies less vigorous, and dangerous 

 fluctuations of temperature far less frequent, in the sea than on land. 

 The same features make the land fauna more clearly indicative of 

 minor divisions of the scale, and of the progress of organic evolution 



