Mammillaria elongata f. cristata is the crested form of this popular species. Often called the 'Brain Cactus' as its stems grow in a crested shape, ultimately resembling a brain.
It is fairly slow growing, but easy enough to grow. As it is a crested form, normal growth can return at any time. If it does, remove this growth from the crested part. It likes a very well drained growing medium and a position with plenty of bright light. It can be placed in full sun, although some relief from the harshest afternoon sun is beneficial. Water only when dry in the warmer months and very sparingly in winter.
With its unique features, this cactus is certainly a statement piece.