Planetoids, the name given to a number of very small planets revolving between the orbits of Mars and Jupiter, originally called Asteroids, all of recent discovery, and the list, amounting to some 400, as yet made of them understood to be incomplete. They are very difficult of discovery, many of them from the smallness of their size and their erratic movements.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Planché, James Robinson * Planets