What is "perischoechinoidea"

Perischoechinoidea is a subclass of primitive sea urchins that were abundant in the Palaeozoic seas. However, the great majority of species died out during the Mesozoic, as the more advanced euechinoid sea urchins became common. Today, only a single order, the Cidaroida, survives.

Most fossil forms had multiple columns of ambulacral plates, rather than the two rows found in all living species of sea urchin. They also lacked a perignathic girdle around the mouth.