Page:The Habitat of the Eurypterida.djvu/165

The physical and faunal characteristics which have just been described have usually been interpreted as indicative of shallow-water marine conditions of sedimentation during the late Siluric. The physical nature of the Temeside group would, perhaps, not preclude such an origin, but the faunal characters leave no doubt that the Temeside group must have been deposited on the land. As is so frequently the case in such successions of sands and flags, there is no doubt that the material is terrigenous in origin, the only question being the place of deposition, whether on the land or along the littoral margin of the sea. If we apply the criteria for the recognition of the various types of fossil faunas, it is at once evident that neither throughout the Temeside group nor at any particular horizon in it is there a marine fauna, for we have seen that a marine fauna, whether existing under the rather uniform conditions of the open sea or under the more variable environment of the littoral zone, and whether in a sandy, muddy, or pure water facies, was yet made up of diverse classes of organisms with a scattering representation through the phyla of the Invertebrata. In the Temeside group there is no bed containing representatives of more than two invertebrate phyla and usually only one phylum is represented. The maximum thickness of this group is 170 feet. In the lower 50 feet (Downton Castle sandstones) the deposits are cross-bedded and contain Lingula minima at certain levels, but no other fossils. Such a series is to be accounted for only by deposition at the mouth of a river, either on the subaërial portion of a delta or on the flood-plain, but the coarseness of the deposits implies that the former was the more probable region of deposition. The presence of beds of Lingulae is easily accounted for by the nature of the shells which are thin, corneous, and consequently of small specific gravity. Exceptionally high tides would easily wash in such light shells far up over the delta, while heavier shells would be dropped farther out in the littoral waters. It is evident that the Lingulae must have been transported from their original habitat since they are unassociated with any other forms of life, unless they can be regarded as living in the river mouths. Thus the assortment seems to have been by specific gravity. In the 120 feet of the Temeside group, Lingula cornea replaces L. minima in the single bands, and is to be accounted for in a similar manner. Now it might be suggested that the eurypterids, which are likewise found in thin bands, were also washed in from the sea on account of their light specific gravity; but the difference between the two cases