Order 1. _Bothriocidaroida._--Ambulacrals simple, each with two pores vertically superposed, 2 columns to each ambulacrum; interambulacrals multi-tuberculate, in 1 column, none passing on to or resorbed by the peristome; mouth central, jaws unknown, no external gills or sphaeridia; anus aboral, endocyclic. Sole genus _Bothriocidaris_ (fig. 5), Ordovician. Order 2. _Melonitoida._--Ambulacrals simple, each with two pores horizontally juxtaposed, in 2 to 18 columns; interambulacrals granulate with occasional tubercles, in 3 to 11 columns, not more than one row passing on to the peristome; mouth central, with jaws, no external gills or sphaeridia; anus aboral, endocyclic. Families: Palechinidae (fig. 19, 1), Melonitidae and Lepidesthidae, Silurian to Carboniferous. Order 3. _Cystocidaroida._--Ambulacrals simple, each with one or two pores, which sometimes pass between rather than through the plates, in 2 columns; interambulacrals, uni- or multi-tuberculate, in numerous (say 10 or more) columns, none passing on to peristome; mouth central with jaws, no external gills or sphaeridia; position of anus doubtful, acyclic, i.e. no apical system so far as known. Include only _Echinocystis_, _Palaeodiscus_ and (?) _Myriastiches_, all Upper Silurian. Order 4. _Cidaroida._--Ambulacrals simple, each with two pores horizontally juxtaposed, in 2 columns; interambulacrals unituberculate, in 2 to 11 columns, some rows may pass on to the peristome; mouth central, with jaws, no external gills or sphaeridia; anus aboral, endocyclic. Families: Lepidocentridae and Archaeocidaridae (fig. 19, 2), Devonian and Carboniferous; Cidaridae (fig. 19, 3, 4). Permian to present; Diplocidaridae and Tiarechinidae, Mesozoic. Order 5. _Diademoida._--Ambulacrals generally compound, with two pores obliquely juxtaposed, in 2 columns as in all subsequent orders; interambulacrals usually with large radioles surrounded by smaller ones, as in Cidaroida, in 2 columns as in all subsequent orders, only one plate resorbed; mouth central, with jaws and external gills, sphaeridia present; anus aboral endocyclic. J.W. Gregory divides this into four suborders, each representing a distinct evolutionary series; i. _Calycina_, Saleniidae (fig. 19, 5) and Acrosaleniidae; ii. _Arbacina_, Hemicidaridae and Arbaciidae; iii. _Diademina_, Orthopsidae, Diadematidae, Diplopodiidae, Pedinidae, Cyphosomatidae, and Echinothuridae; iv. _Echinina_, Temnopleuridae, Triplechinidae, Strongylocentrotidae and Echinometridae. The order is Triassic to Recent. [Illustration: FIG. 19.--Denuded tests of some fossil Echinoids. 1, _Palaeechinus_; Carboniferous. 2, A plate and radiole of _Archaeocidaris_; Carboniferous. 3, A radiole of _Cidaris_; Jurassic. 4, _Hemicidaris_; Mid. Jurassic. 5, _Salenia_; Cretaceous. 6, _Dysaster_; Jurassic. 7, _Enallaster_: Cretaceous. 8, _Catopygus_; Cretaceous.]