Mammillaria decipiens - Bird's Nest
Long, soft tubercles with a few short spines on the ends. Can handle lower light than most cacti, likes bright indirect light and morning sun only. In the wild it grows under the shade of other plants. Puts out offsets from the base and can eventually form a clump. Thick tuberous roots. Puts out pretty little white flowers. Very sensitive to over-watering. Use very well draining soil mix, and keep it dry in the winter.
Current batch is LAST 3 pics. These are little and very cute!