In 1874 he predicted that the ancestor of all the mammals would be a five-toed, flat-footed walker, with tubecular molar teeth, or in exact language, a pentadactyl, plantigrade bunodont. Seven years after, he obtained evidences that such a type of mammals abounded in North America during the early Eocene Tertiary period. Professor Cope,[1] in his phylogeny of the camels, shows a remarkable parallel to that of the horse, both forms appearing in the Lower Eocene. Mr. Eugene N. S. Ringueberg[2] believes he has found in a thin layer of limestone at Gasport, New York, a deposit in which a number of forms of brachiopods seem to present the intermediate stages between certain brachiopods common to the Clinton and the group of rocks immediately above. While the majority of species in this deposit belong to the Niagara, there are among the fossils met with three species of brachiopods which were supposed to have passed out of existence with the Clinton. He finds in this bed thirty-two forms peculiar to the Niagara, eleven common to Niagara and Clinton, three belonging to the Clinton, and two characteristic forms of the transition group. Many of these show intermediate characters.
Professor II. S. Williams,[3] in his paleontological studies of the life history of Spirifer lævis, in which he traces the ancestral line of this creature, says: Whatever theoretical description we may give to species, here are, in the first place, an abundance of individual organisms whose remains are found in the Upper Silurian rocks of Europe, Great Britain, and America, presenting a few clearly marked, distinctive characters, which are found variously developed in the individual forms, but so grading in the various varieties as to cause careful naturalists to associate them as varieties of a single species."
Dr. C. A. White,[4] in his comparisons of the fresh-water mussels and associated mollusks of the Mesozoic and Cenozoic periods with living species, expresses his belief that the present Unios of North America, particularly those forms allied to Unio clavus, have come down in an unbroken line from the Jurassic and possibly from earlier times. He shows that thus far all the fossil Unios have been obtained from lacustrine deposits, none of these beds being distinctly fluviatile. He furthermore calls attention to the fact that "these lacustrine formations are of very great extent in Western North America, and, without doubt, the lakes in which they were deposited were caused by encircling bands of rising land during the elevation of the continent. These great landlocked waters were at first brackish, but finally became, and for a long time remained, fresh, continuing so until their final desiccation." From this commingling of salt and fresh water he justly assumes that many modifications arose in the forms of Unios
