Page:Popular Science Monthly Volume 76.djvu/587

Rh parts were supported or protected by a stony skeleton of some kind; no parts of the skeletal structure were too minute to be kept practically unaltered to the smallest microscopic detail; no period of time has been so long that the records of the large or the small things of life were necessarily obliterated. The shells of such minute and delicate things as radiolaria and foraminifera, on the one hand, and that king of invertebrates, the giant Camaroceras, on the other, have all been kept through the ages with equal fidelity. The hinge characters of the brachiopods, their internal arm supports, their spires and loops, the distribution of the ramifying blood channels in the mantle, the surface markings of every rank and grade down to the smallest which can be observed only with the lens, and the microscopic structure of the shell itself, are other examples of the faithfulness with which details, however insignificant in point of magnitude, have been guarded, protected, preserved. Strangely enough, in respect to a very large proportion of the animal remains buried in the ancient sediments it looks as if time had been standing still; it has neither marred nor destroyed. The organic remains from the Ordovician formations are quite as perfectly preserved as those from the Tertiary.

The profusion of the life of the ancient seas is as much a source of surprise as the detailed perfection of the record. In the Mississippi Valley limestones constitute a very large proportion of the sedimentary rocks, and it is unnecessary to say that these limestones record the life and death of countless myriads of organisms. In some cases the waters of the old seas were comparatively quiet, and the shells or other hard parts, undisturbed and unbroken, remain in the positions they occupied when the individuals they represent were alive. There is a bed of marly shale carrying many thin lenses of limestone, lying between the Platteville and the Galena, from 60 to 70 feet above the base of the Mohawkian, and these lenses are made up in large part of unbroken brachiopod shells. On the surface of one of these slabs, in an area measuring 35 square inches, one may count more than 60 perfect specimens of Dalmanella subæquaia and Orthis tricenaria. The rate is about 290 individuals to the square foot. The number on a square mile of such sea bottom runs up into the billions. The number of individuals of the species Pentamerus oblongus that swarmed on the bottom of the Niagaran sea is strikingly demonstrated in every paleontological museum. The wide geographic range of this species is well known; its range in time was such as to make possible the accumulation of beds of limestone, 70 feet in thickness, from the detritus of its broken shells. Like other persistent or widely ranging species, it gave rise to a very large number of varietal forms, some of which have been described as specifically distinct.

The Devonian formations furnish similar evidence of the