clover fern: any of several water ferns of the genus Marsilea having four leaflets
fern: any of numerous flowerless and seedless vascular plants having true roots from a rhizome and fronds that uncurl upward; reproduce by spores
post-rotational nystagmus: nystagmus caused by suddenly stopping the rapid rotation of the body; large slow movements of the eyeballs are in the direction opposite to the direction of rotation