RODENTIA.
This order includes an extensive series of small Mam mals in which a single pair of large, curved, ever-growing incisors in each jaw is associated with many other peculia rities of structure. These incisors (fig. I 33, sepa rated by a wide interval from a short series of molars, characterize the whole order of Rodents ; the single exceptional family, Leporidce, includ ing hares, rabbits, and Picas or tailless hares of Siberia, retaining a second minute incisor behind each of the larger ones in the upper jaw.
The small size of the great majority of the species of this order leads to the neglect or the oversight of their fossil remains by the labourers in quarries and other deposits of stone, to whom the paleontologist is usually indebted for his first acquaintance with characteristic fossils of such formations. No evidence has yet been obtained of any unequivocal remains of a rodent animal in strata more ancient than the eocene tertiary deposits. Cuvier detected remains of Rodents allied to the dormouse (Myarus) and squirrel (Sciuru,$) in the eocene building-stone of the Mont martre quarries near Paris. The lacustrine marls of the middle (miocene) tertiary period have yielded evidences of not fewer than eleven genera of Rodentia distinct from any now known to exist. The deposits at Eppelsheim, near Darm stadt, of the same miocene age, have given evidence of Rodents akin to the marmot and the beaver. The more recent tertiary formations and the bone-eaves in have furnished fossil remains not distinguishable from the existing beaver, hare, and rabbit, water-vole and field-vole, as well as remains of a Pica, or tailless hare, belonging to the genus Lagomys, now confined as an existing species to Asia ; and of a very large Rodent, akin to the beaver, called Trogon therium. Similar fossil remains have been abundantly found in
the pliocene and pleistocene formations of continental Europe, including representatives of the genus Hystrix, or fossil porcupines (11. refossa, Ger.), from the pliocene of Issoire (fig. 133). The coeval deposits of America have yielded fossil remains of extinct species belonging to genera—e. g., Lago seams, Echimys, Ctenomys, Ccelogenys, and other Cavies now restricted to South America. In North America, fossil remains of a Rodent of comparatively gigantic size have recently been discovered. Some parts of the skeleton, and more especially the dentition of the rodent order, are highly characteristic—the form of the articular surface for the lower jaw, which is a longitudinal groove, the molars, especially of the phytiphagous kinds, crossed by enamel plates more or less transverse—these, with the long, curved, chisel-shaped incisors, two in each jaw, suffice to determine the ordinal relations of the fossil. The incisors alone would not be always so safe a guide, for the rodent modification of these teeth is repeated in the marsupial wombat and the lemurine aye-aye.
With regard to the Rodentia, the great beaver (Trogon therium) seems to have become extinct in England and the Europteo-Asiatic continent before the historical period ; whilst the smaller pliocene beaver continued to exist with us, like the wolf, until hunted down by man. It still survives in a few of the great continental rivers. Of the little Lagomys of our ossiferous caves no living example remains in either England or Europe. The species, indeed, may be extinct : its genus is now limited to Central and Southern Asia.