are either incorporated in, or pressed below, the tegmen by interambulacrals; all thecal plates united by suture, somewhat loose in the earliest forms, but speedily becoming close, and producing a rigid theca; mouth and tegminal food-grooves closed; arms pinnulate.

All the Palaeozoic representatives have non-pinnulate arms, while the Mesozoic and later forms have them pinnulate.

are either incorporated in, or pressed below, the tegmen by interambulacrals; all thecal plates united by suture, somewhat loose in the earliest forms, but speedily becoming close, and producing a rigid theca; mouth and tegminal food-grooves closed; arms pinnulate.

All the Palaeozoic representatives have non-pinnulate arms, while the Mesozoic and later forms have them pinnulate.