Mammirraiia zailmanniana is an easy and common plant, cultivated for it's nice and easily produced dark-pink flowers. It is a pretty much a 'beginer's cactus'.
The plant pictured is about 10 years old from seed, and for the last 2-3 years it develops this growth abnormality called 'cristate'. The grow point of the main stem turned into grow line, that keeps getting longer and longer with time. This does not stop Mammillaria zailmanniana from blooming, and now instead of rings of flowers this plant has twin ribbon of flowers.
This 'cristate' growing is relatively uncommon but due to the special interest by the fascinated cactus lovers many forms like that are propagated in collections. In most cases cristate plants are grown on graft. This plant I have obviously does not require that.