species. The presence here of heavy beds of conglomerate with very thin shale seams between corroborates the view that these forms were but little adapted to digging in the mud. The prevalence of Stylonurus in the fauna would seem to support Laurie's suggestion that Stylonurus possessed purely littoral habits.
The sedimentary facies in which the Otisville and Schenectady faunas are involved is not a usual accompaniment of the eurypterids, as fine mud rocks constitute the prevailing sediment and relatively broad-headed forms the expression of the body.
If one tries to picture the group as a whole, the typical habit would appear to be that of the mud grubber, and the broad carapace, relatively broad preabdomen, the flippers and the tail spine will be the most important elements in producing this picture.
It is very interesting to note in this connection that a subclass of an entirely different phylum, viz, the Ostracophora (Cephalaspis, Pteraspis [Old Red Sandstone of Scotland]), among the fishes, lived at the same time with the eurypterids, is frequently associated with them in the rocks, had acquired the same mud groveling habit and a similar general form. The theory that these earliest fishlike vertebrates are derived from the arthropod stem, and have features in common with the merostomes (eurypterids) and arachnids (scorpions etc.) is still seriously defended by some competent investigators. Eastman, in an excellent essay[1] on the evolutionary history of the fishes, has emphasized the fact that the merostomes and arachnids at this early date had already diverged too widely along certain directions from the primal trilobitic type of organism, to be the possible ancestors of backboned animals, and such resemblances as are shared by merostomes and early fishlike vertebrates are explained as "due to mimicry, or to adaptation of creatures of different grades to a similar environment."
We are here not so much interested in the problem of the possible derivation of the vertebrates from the merostomes, as in the fact of the
- ↑ C. R. Eastman. Iowa. Geol. Sur. Rep't 1908. 18:51f.
