the Niobrara Valley, Dr. Leidy describes a rhinoceros so like the Asiatic species, R. Indicus, that he at first referred it to the same, and, what is most singular, he remarks generally of the Pliocene fauna of that part of North America, that it is far more related in character to the post-pliocene and recent fauna of Europe than to that now inhabiting the American continent.
It seems indeed more and more evident that when we speculate in future on the pedigree of any extinct quadruped which abounds in the drift or caverns of Europe, we shall have to look to North and South America as a principal source of information. Thirty years ago, if we had been searching for fossil types which might fill up a gap between two species or genera of the horse tribe (or great family of the Solipedes), we might have thought it sufficient to have got together as ample materials as we could obtain from the continents of Europe, Africa, and Asia. We might have presumed that as no living representative of the equine family, whether horse, ass, zebra, or quagga, had been furnished by North or South America when those regions were first explored by Europeans, a search in the transatlantic world for fossil species might be dispensed with. But how different is the prospect now opening before us! Mr. Darwin first detected the remains of a fossil horse during his visit to South America, since which two other species have been met with on the same continent, while in North America, in the valley of the Nebraska alone, Mr. Hayden, besides a species not distinguishable from the domestic horse, has obtained, according to Dr. Leidy, representatives of five other fossil genera of Solipedes. These he names, Hipparion, Protohippus, Merychippus, Hypohippus, and Parahippus. On the whole, no less than twelve equine species, belonging to seven genera (including the Miocene Anchitherium of Nebraska), being already
