- Ambulacrals simple, each with one or two pores, which sometimes pass between rather than through the plates, in 2 columns; interambulacrals, unior multi-tuberculate, in numerous (say io or more) columns, none passing on to peristome; mouth central with jaws, no external gills or sphaeridia; position of anus doubtful, acyclic, i.e.
- Ambulacrals sometimes compound, with one or two pores to a plate, some dorsal podia begin to assume respiratory function; interambulacrals multi-tuberculate, none resorbed; mouth central, with jaws weak or wanting, with external gills and sphaeridia; anus exocyclic. Families: Pygasteridae, Discoidiidae, Galeritidae, Conoclypeidae; Jurassic to Recent.
'S ' --endocyclic jaws flat-reduced - lost branchiate ' 1 - lipobranchiat e sphaeridia exocyclic jaws loft y - abranchiate - ?
Lno sphaeridia J Book of Palaeontology (New York and London, 1899).