The positive image on the left would, I had thought, be properly described as being full of holes. Inverting the image shows that in fact the Latin description "full of hollows" is more nearly correct.
This is an unusual image of the lacunosus formation in low level cumulus fractus cloud. Normally the formation is seen in cirrus and occasionally in mid level cloud. See en.wikipedia.org/wiki/Cirrocumulus_lacunosus
From the Latin "Full of hollows".
Fossil wave cloud lower middle.