2o8 THE HABITAT OF THE EURYPTERIDA 



4. In deposits which, from the study of their lithogenesis can be 

 shown to have come from the same Palaeozoic continents, should be 

 found remains of eurypterids in circumscribed areas as stated in "2" 

 above, and the genera and species, while not necessarily having any 

 near relatives in adjoining deposits, may be identical with forms whose 

 remains are found in a formation perhaps two or three thousand miles 

 distant, but on the same ancient continent. Such relationships are to 

 be accounted for by migration from a common source where the head- 

 waters of two or more river systems interlace (see p. 205 above). 



5. The distribution of eurypterids would not have had any neces- 

 sary connection with those organisms living in marine chronofaunas, 

 and consequently, except when eurypterid-bearing deposits merge 

 into thalassigenous ones, or when fragments or stray eurypterids have 

 been washed out to sea, when intercalation between marine deposits 

 would give the age, eurypterids would not serve as good index fossils. 



6. Eurypterids would not suffer rapid changes in evolution, since 

 it is a well known fact, that fluviatile types are often persistent for 

 a long period of time. Thus the cray-fish Cambarus pHmaevus 

 Packard of the Green River beds (Eocenic) of Wyoming, is a near 

 relative of the modern C. affinis of the same region, a similarity due no 

 doubt to the persistence of the type in essentially the same river basin 

 during the interval. 



Zoologists and palaeontologists who have made detailed studies of 

 the distribution of modern freshwater faunas are thoroughly agreed 

 that accurate results are not to be obtained merely from observations 

 on present distribution. It is an absolute necessity to study the fos- 

 sil faunas and especially the palaeogeography. The reason for this 

 will be evident after a very little thought. If in the Lower Cretacic 

 when there existed the Nearctic continent, comprising most of North 

 America, and continuing across the North Atlantic through Green- 

 land and western Europe, and including the Scandinavian mass, a 

 family of some fluviatile organisms had arisen in the central Canadian 

 area, quickly spreading from one river system to another and finally 

 reaching Europe, we would find in the rocks of that period, that many 

 of the genera on the two modem continents were the same, and that 

 there would be quite a goodly number of identical species. The de- 

 scendants of these Lower Cretacic organisms would develop on the 

 two continents, (i.e., the two sides of this old nearctic land mass), 

 and the species in the lower reaches of the rivers would diverge in 

 their characters more and more from the parent stock. Those forms 



