clypeastridæ

from The Century Dictionary.

• A family of irregular seaurchins, flattened into a discoidal or shield-like shape, with the mouth central and furnished with a masticatory apparatus; the shield-urchins.
• In Gegenbaur's system of classification, a group of petalostichous Echinoida, represented by the genus Clypeaster and its relatives, as distinguished from the spatangoid sea-urchins. Also Clypeasteridæ, Clypeastroidea.