Some light seems also to be shed on their probable mode of life by a consideration of the separate faunules to which they pertain. Thus the Otisville fauna is composed of notably slender forms, such as Hughmilleria shawangunk and Eurypterus maria, which indicate that a large element of the fauna consisted of agile, swimming
as Pterygotus and Hughmilleria. We have, however, been impressed from another consideration with the possibility of their having been able to swim on their backs. In casting about for living water animals of a shape similar to the general expression of the eurypterids, we could find no better analogy than the well known "water-boatmen" or "boat-flies" (Notonecta) of our ponds with their elongate elliptic bodies, anteriorly rounded carapace with large marginal eyes, and long natatorial feet in the middle of the body. We have again and again been struck with this analogy in looking over the young specimens of Eurypterus remipes with their outstretched swimming feet, or in wondering at the long and powerful oarlike feet of Dolichopterus. The species of Notonecta swim on their backs, but those of Corisa, a closely related genus of associated waterbugs (the "oar-feet bugs") of like outline as seen from above, swim like all other water insects on their ventral surface. It appears that the peculiar attitude of Notonecta is principally adapted to the easy accumulation and storage of air in the hairy covering of the upper (ventral) side and that the body is distinctly boat-shaped, the dorsal side being keeled, to facilitate the inverted mode of locomotion, while in Corisa as in the other swimming waterbugs the body is distinctly flat.
From analogy with Notonecta and Corisa we consider it possible that Pterygotus and other types of eurypterids could have assumed an inverted position in swimming, but there are several reasons why it is unlikely. The most important of these is that the inverted attitude is not the normal one for nektonic organisms, but distinctly an adaptation to special conditions. It is not apparent that any of these conditions existed in the adult eurypterids, and the marginal eyes of the Hughmilleria and Pterygotus group would permit, by analogy with Notonecta and Corisa, either attitude in swimming.
In regard to the probable swimming attitude of the long-legged eurypterid types, Drepanopterus and Stylonurus, we know no better analogy than the water spider of the middle European ponds (Argyroneta aquatica). Any one who has watched the ease with which this interesting arachnid swims by means of its long spider legs and even overtakes its prey of water insects, can not fail to appreciate the swimming possibilities of even the awkward looking Stylonurus. This water spider also differs from other spiders by having the median eyes raised on a round mound, and the lateral ones on oblique nodes, just as in the eurypterids and especially in Stylonurus. The water spider swims with its right (dorsal) side up, and by analogy we infer that the similar Stylonurus group did the same.
