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238 that of the latter. The similarities, however, are pronounced, and it is not to be denied that P. osiliensis finds its nearest relative in P. bilobus. Aside from this species, there are several others belonging to the subgenus Erettopterus, and we cannot dismiss our comparative study without calling attention to them. They are: Pterygotus (Erettopterus) grandis, and globiceps from North America; but they are so very distinct from the Baltic form that genetic relationship is in no way indicated.

The variety laticauda of P. osiliensis is founded not without certain misgivings on the part of Schmidt for an exceptionally large metastoma and a similarly large telson found associated with the P. osiliensis specimens. So far as the present problem of the determination of the relations between faunas is concerned, this variety would be classed along with P. osiliensis, and needs no separate discussion.

The species of faunas of the Upper Siluric waterlimes of Oesel, Gotland, Livland, Podolia, and Galicia are thus seen to show very close relationship either to species in the Bertie waterlime, as in the case of Eurypterus fischeri, and var. rectangularis, or to species found in Great Britain at the end of the Siluric or the beginning of the Devonic. That is, they show affinities to the faunas occurring in deposits which for reasons other than faunal ones were judged to have been derived from the continent of Atlantica.

The Fauna of the Wenlock. There now remains only the discussion of the eurypterid-bearing deposits of Great Britain, (6) the Wenlock of Scotland, and (7) the Old Red sandstone. In the Wenlock beds there is a large fauna represented by at least twelve determinable species of eurypterids, and one would expect to be able to attain to some critical knowledge of the relationship of the forms there occurring to those in North America and Europe; but while the fauna lacks not in the number of species and of individual remains, complete or even nearly complete specimens are not to be found, and one is forced to attempt to draw conclusions concerning relationships from fragments of legs, carapaces, or body segments, an attempt which is not only difficult but altogether unsatisfactory because of the probable errors attending it. Let us, however, consider the species