198 THE HABITAT OF THE EURYPTERIDA 



directly associated with the marine forms. From a study of the 

 material in the Palaeontological Museum at Columbia University, 

 I have found that the rock in which Strabops occurs is not of. the 

 same lithological character as is that in which the other fossils occur. 

 The slab on which the counterpart of Strabops rests is roughly 

 3x9x12 inches in dimensions and contains no other organic remains. 

 The limestone containing the trilobites is somewhat finer grained, 

 differing little in color, but being made up of numerous cephala, 

 pygidia and fragments of several genera of trilobites. The difference 

 in faunal character between the two rocks is pronounced. The slab 

 containing Strabops was not collected by the same person nor at the 

 same time as were the other fossils, so that exact data probably will 

 never be obtained. However, the precise association is of slight 

 import. The alternation of limestone conglomerates and shales in 

 the lower Potosi series indicates near-shore conditions of sedimenta- 

 tion, and the occurrence of the single specimen of a eurypterid, far 

 from pointing to a marine habitat for this one individual, militates 

 very strongly against such a mode of life. In all cases the occurrence 

 of a single individual is one of the strong arguments against the 

 assumption that the individual belongs to the fauna of the bed in 

 which it is found. It is far more logical to assume that it has been 

 brought there by some accident, for in Nature we do not find single 

 individuals of any kind of animal in a region far removed from that 

 occupied by other members of its family. Again, the only way to 

 account for this occurrence is to assume that these eurypterids were 

 living in the rivers of that time, and that this individual happened 

 to be carried out into the shallow sea in which the Potosi limestone 

 was being deposited. That the sea was shallow is indicated by the 

 fine stratification of the rock as well as the paucity of the organic 

 remains which are insufficient to have furnished the lime of which 

 the formation is composed. This limestone like others of its kind 

 seems to have been formed from the calcareous sand and mud carried 

 by surcharged rivers coming from limestone regions into shallow sea- 

 border basins. 



The merostomes of the Stephen shale of British Columbia are 

 not now recognized as eurypterids, but belong to a distinct order, 

 that of the Limulava Walcott (Clarke and Ruedemann 39, 410). 

 Hence their association with marine organisms may be disregarded. 



3. It may not be quite so clear that the occurrence of a fairly large 

 fauna of eurypterids in a bad state of preservation, but associated 



