This grower is sure of this plant identification.
2 year old specimen of Astrophytum myriostigna in a 1.5 in diameter vintage terracotta pot.
Bishop's cap cactus, Bishop's hood, Bishop's hat
Astrophytum myriostigma (Bishop's cap cactus). This is a rounded cactus green-grey with white scales. The flowers are yellow and glossy and bloom in the summer.
They are frost tender with a min of 5C. They will prefer sun and very well drained soil, lime rich soil. It is rather prone to rot if it becomes too wet. Dry completely in winter.
Grown for their freely produced, flattish, yellow flowers, some with red centres.