Paleontologists say they have discovered ten new prehistoric rodent species found at the John Day Fossil Beds National Monument and on nearby lands.  The current issue of the Annals of Carnegie Museum reports Dr. Joshua Samuels of the John Day Fossil Beds National Monument and Dr. William Korth of the Carnegie Museum of Natural History examined newly discovered and previously undocumented fossil specimens from the John Day Formation.   Their study describes 21 species of rodents in all.   The new species include: an early beaver which may be the distant ancestor of living beavers, a dwarf tree squirrel smaller than any living in North America today, a primitive pocket mouse, and a birch mouse.  Some of the new rodents are closely related to species from the fossil record of Asia, and help document the dispersal of species across the Bering Land Bridge.