82 THE HABITAT OF THE EURYPTERIDA 



recognized in ancient deltas by a lithological and faunalinterfingering. 

 The silts brought down by the river will contain the remains of the 

 river fauna, while the submarine deposits, be they sandstones, shales 

 or limestones will contain a marine fauna. The two types of depos- 

 its as well as the two types of faunas, though they interfinger, will be 

 of a distinct and recognizable character as a rule, and the nature of 

 the faunas, in regard to numbers of individuals and species, will be 

 recognized by the characteristics listed on page 77 above. The 

 deposits on the subaerial portions of the delta and laterally in the 

 flood-plain areas will consist of fine silts. Along the river banks the 

 coarsest of the silts will be deposited and with them the heavier or- 

 ganic remains if there are any, such as the shells of molluscs. But 

 with the finer silts periodically spread out at times of flood far to 

 either side of the river, will be carried only the lightest materials, 

 probably only plant remains and the exoskeletons of the various fluvi- 

 atile crustaceous animals. Such organic remains may be carried out 

 in great numbers, and if quickly buried will be excellently preserved 

 in the fine muds. On the other hand, if they are carried a long dis- 

 tance, dropped and exposed to the air, and later perhaps picked up 

 by some distributary and carried on again, the process being often 

 repeated, they may be broken up, and when they finally come to rest 

 and are buried, not a single complete organism will remain. Indeed, 

 if in their final resting place they are exposed to the air for a long 

 time and the mud on which they lie becomes sun-cracked, the frag- 

 ments, drying up, may be blown for great distances, perhaps far in- 

 land or perhaps out to sea, coming to rest at last in regions far removed 

 from those in which the organisms had lived and there amidst a strange 

 fauna the remains may be entombed. 4 In the sediments such a his- 

 tory could be read, if in shales or waterlimes the only organic remains 

 were those of light specific gravity. Of the invertebrates there would 

 probably be some arthropods or insects; among the plants, leaves, 

 algae, reeds and grasses would be expected. Such a deposit would 

 be difficult to correlate with a marine deposit, because it might con- 

 tain none of the contemporaneous marine organisms. If, perchance, 

 some of the river organisms or fragments of them had been blown or 

 carried to sea, their remains could be entombed with the typical ma- 

 rine fauna and the age would thus be determinable. It is not un- 

 likely, too, that stray molluscan shells might be blown from the shore 



4 In this connection it is interesting to record that the eurypterids of Oesel have such a thin 

 test, that specimens exposed by the breaking of the rock are not uncommonly blown away by the 

 wind. 



