THE EURYPTERIDA OF NEW YORK 83 
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 Hugh - 
milleria 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-boat- 
men ”’ 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 asso- 
ciated 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. 
