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198 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 3 x 9 x 12 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 sedimentation, 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 seaborder 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 hot be quite so clear that the occurrence of a fairly large fauna of eurypterids in a bad state of preservation, but associated