Page:The Habitat of the Eurypterida.djvu/17

 course of which he may find better preserved fragments in the Belt terrane, leaving no doubt as to the nature of the organisms; or, he may find other structural and stratigraphic evidence for the correlation. "On the basis of lithologic characteristics," he says, "the Altyn would be correlated with the Newland limestone, and the Grinnell and Appekunny with the arenaceous series above the Newland limestone." But he further points out that "In deposits of the character of those of the Algonkian in Montana, lithologic characteristics are really of very little value over extended areas, as most of the calcareous formations are in the form of great lentils, and these are not comparable with the calcareous deposits of the Palaeozoic."

. In the Middle Cambric there are undoubted marine Merostomata, discovered by Walcott in 1910 in the Stephen formation in British Columbia, Canada. He has described two genera, Sidneyia and Amiella, referring them to the Eurypterida in the sub-order Limulava. As will be shown later, these forms are not true eurypterids, and need, therefore, no further mention here.

The only unquestionable eurypterid from the Cambric is Beecher's Strabops thacheri from the Potosi limestone at Flat River, St. François county, Missouri (19, pl. VII). Of this species a single specimen was found for which the genus was erected. It is a nearly complete individual, the dorsal aspect of which is well shown, though none of the appendages are visible. It occurs in a yellowish, argillaceous calcilutyte from one of the lower members of the Potosi. The slab upon which Strabops occurs contains no other organic remains, but Beecher has described a collection made by Nason from these same beds in which there is an abundant marine fauna consisting of fragments of trilobites with a few brachipods and other forms (Hyolithes and a small Platyceras) (20, 362, 363). It is to be regretted that Beecher did not, or was not able to specify more exactly the stratum in which he found the eurypterid, for the Potosi limestone in the Flat River section is 350 feet thick, not counting the 106 feet of slates and conglomerate below and another 100 above, all of Potosi age, and of course, it is by no means certain that the marine fossils occurred in the same bed with the eurypterid. In fact, so far as the material is concerned, this seems not to have been the case.

. From the Ordovicic until just recently only one occurrence had been noted, that of Echinognathus clevelandi Walcott,