cystocidaroida

from The Century Dictionary.

• An order of primitive Palæechinoidea regarded as transitional to the Cystoidea and characterized by an exocyclic test, narrow and straight ambulacra, with 4 vertical rows of plates each with a central pair of pores, broad interambulacra, numerous scale-like echinate plates, periproct posterior, and mouth with well-developed jaws. It is of Silurian age.