104 THE HABITAT OF THE EURYPTERIDA 



Sarle to be a plant. Upon this bed lies the shaly marlite, the first 

 of the lowest Salina or last of Upper Niagaran, 5 and above this a 

 waterlime. 



There is thus a marked change in the physical conditions which 

 accompanied the withdrawal of the Niagaran sea and the initiation 

 of the Salina type of deposits. In the lowest bed of the series occurs 

 the last of the Niagaran fauna, Pterinea cf. emacerata. In the water- 

 lime (no. 14) the same species occurs, together with a Lingula, 

 Leper ditia cf. scalaris and an Orthoceras. Then not again till the 

 black shale of the eurypterid horizon, do these forms appear, but it 

 is of great significance that these pure marine fossils do not occur in 

 the eurypterid shale beds proper, but in the thin dolomite partings, 

 and that in these partings the eurypterids are almost entirely wanting. 

 The dolomites were evidently marine, but the conditions under which 

 they were deposited "were not favorable to the eurypterids" says 

 Sarle (240, 1086). The question immediately arises: why were the 

 conditions not favorable? and then, where were the eurpterids during 

 the intervals between shale deposition, i.e., during the time of marine 

 dolomite deposition? If the eurypterids were inhabitants of the 

 marine waters or of bays, estuaries, etc., they should be found in the 

 beds containing the littoral fauna of the impure dolomite, for, as has 

 been shown in Chapter III, the faunas of bays and other indentations 

 along the shore are not restricted to such areas, but are much the same 

 as the faunas extending all along a continental shore line. Since the 

 eurypterids were not living in the marine waters, where were they 

 when there were no black muds forming? They appear suddenly in 

 countless numbers representing six species, and they come with the 

 black shale facies and disappear again just as suddenly. Their 

 range, too, is very small. Sarle says: "Though the fine character 

 of the silt forming the black shale and the evidence of interrupted 

 sedimentation noted above, indicate slow accumulation, the occu- 

 pation by the eurypterids was apparently of comparatively short dura- 

 tion, merely an incursion, as it were, since the black shale all told does 

 not exceed 2 feet in thickness" (240, 1086). 



To determine where the eurypterids were before their two sudden 

 appearances, one must turn to the source of the black muds. These 

 were not deposited in the open sea, for marine forms are wanting, 

 and in any case, the mud must have been derived from the land. 

 At that time the land to the west, north and south was all covered 



5 Professor Grabau has recently voiced the opinion, that the Pittsford shales and Shawangunk 

 conglomerate are better considered as the closing deposits of the Guelph period. 



