Full Idea
'Plane' is the genus of figures in the plane, and 'solid' is the genus of solids. For every figure is either a plane with abc features or a solid with xyz features, so it is the plane and solid which underlie these differentiae.
Gist of Idea
'Plane' is the genus of plane figures, and 'solid' of solids, with differentiae picking out types of corner
Source
Aristotle (Metaphysics [c.324 BCE], 1024b01)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.147
A Reaction
Thus you could hardly define a square by merely mentioning that it is a 'plane figure', and you would need pretty precise differentiae before you could be certain you were only dealing with a square, and not a parallelogram.