which may occur among fluviatile organisms, but are inexplicable for marine forms. In the upper Niagaran in North America H. socialis occurs by the hundred in the Pittsford shale and the closely related H. shawangunk, which may be only the young of the former species, occurs in the synchronous shales of the Shawangunk, but in no other part of the world at that time, so far as we know, were there any representatives of Hughmilleria. It appears that the genus originated
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| Fig. 26. Eurypterus lanceolatus Salter. ✕ . (After Woodw. 1878, p. 142, fig. 44) |
Fig. 27. Hughmilleria socialis Sarle. ✕ . (After Cl. & R. 1912, pl. LIX, fig. 1) |
in the rivers of Appalachia. Curiously enough, in the Upper Ludlow, that is, lower Upper Siluric, of Scotland, Eurypterus lanceolatus appears, showing a striking resemblance to Hughmilleria socialis. The prolific Scottish fauna of the Wenlock has revealed no possible ancestors for this distinctive Eurypterus (or Hughmilleria?) and one naturally wonders how it arose. Since Hughmilleria was restricted in occurrence in the Niagaran, any migrations which took place must have been effected during the Salina period. The important Salina
