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Dinomyidm, and Caviidm, containing what are usually called the Chinchillas, Agoutis, Giant-mice, and Cavies, are entirely restricted within its limits, while a fifth family, the Erethizontidm, or Tree-Porcupines, has a single genus in the Nearctic Region.
The Octodontidm, a large group of seventy or eighty species, divided into some twenty-two genera, are also mostly Neotropical, but four peculiar types, Ctenodactylus, Massoutiera, Pectinator, and Petromys forming a little group by themselves, are Ethiopian. The true Porcupines, Hystricidm, of which three genera are known, are found in the Ethiopian, Oriental, and Palaearctic Regions, typical Hystrix being the only one met with in Europe and Northern Asia.
Finally, at the close of the long series of Rodents, we have the two groups of Pikas and Hares, markedly differing from the nineteen previous families in their dentition, and therefore assigned to a separate Sub-order of Rodents as Duplicidentati. The Pikas (Ochotoma), of which some sixteen species are recognized, are restricted to the highlands of the Nearctic and Palaearctic Regions. The Hares (Leporidm) have a much wider distribution, having representatives in every part of the world's surface except in the Australian Region and Madagascar. Of Lepus proper some sixty species are now recognized, the greater number of which occur in the Palaearctic and Nearctic Regions, whilst they are generally scarcer further south, though well represented in Africa
Section V.—Summary And Deductions
Table of genera of Rodents, showing the geographical distribution of the species.