Page:The Habitat of the Eurypterida.djvu/143

Rh textbooks and by authors generally. The main statement that is dwelt upon insistently, and which is so dangerously plausible, is that in the Wenlock the eurypterids are found in an undoubted marine formation in association with typical marine fossils. Such a statement is full not of truths, but of half truths, and these are far harder to combat than actual untruths. In the present instance we have only to consider how much weight would attach to such a statement if the following significant bits of information were added: (1) The eurypterids do not occur in the same beds with the undoubted marine forms, but always in certain leaf-thin bands which carry no other fossils except Dictyocaris ramsayi, a form which may be a fluviatile if not a terrestrial plant or animal, but the systematic position of which is at present wholly undetermined. The thickness of all the beds containing the eurypterid-bearing bands is only one foot; it is a grit and greywacke; the typical marine fossils are found in the shales and in limestone lenses. (2) Not a single complete eurypterid has been found among the hundreds of specimens collected and the very species described by Laurie have usually been founded upon fragments. (3) The exoskeletons have not only been dismembered—this might be expected in any case, for during the process of decay the various members might fall apart and thus be embedded in the mud, few complete individuals being entombed—but the fragments are macerated, the edges are broken and worn, the surface sculpturing indistinct, altogether showing evident signs of wear. (4) The occurrences are not widespread; although many good sections are obtainable in the Pentland Hills inlier, in only two places are eurypterids found. In both cases the remains are confined to bands a few inches thick, extending laterally but a few feet. (5) The eurypterid fauna appearssuddenly with no forerunners and no descendants so far as may be judged from the faunas of the beds immediately below and above the bands containing eurypterids; two of the five genera are confined to this horizon. (The full significance of statements (4) and (5) will be taken up in the next chapter on distribution.)

These are the facts which are generally not mentioned when the eurypterids are declared to be abundantly represented and associated with a good marine fauna. These five facts seem to be difficult of explanation if it be supposed that the eurypterids were marine organisms. How are we to account for the fact that the merostomes accompanied by that form which so often occurs with them, Dictyocaris ramsayi, are found in layers separated from those containing