These are a few of my overwintered Tephrocactus alexanderi var. geometricus DJF319 seedlings, still in a state of dormancy. They have shrunk down considerably from their summer selves.
Once they are ready to wake up in a few months, I'll water them and they will plump up again.
Tephros are some of my favorite cacti. When mature they will look like miniature soccer balls stacked on top of one another.
Some of these cacti can be nearly spineless. Below are a couple of links to a few pics of some mature specimens.