Order 6. _Holectypoida._--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. Order 7. _Spatangoida._--Ambulacrals simple, with two pores juxtaposed, dorsal podia respiratory; interambulacrals bearing numerous small spines, none resorbed; mouth central or shifted forwards, with no jaws or external gills, sphaeridia numerous; anus exocyclic. As the mouth moves forward and the anus downward, the posterior interambulacrals between them are enlarged and strengthened so as to form a sternum. The order may therefore be divided into: (i.) _Asternata_, Famm. Echinoneidae, Nucleolitidae and Cassidulidae (fig. 19, 8); (ii.) _Sternata_, Famm. Collyritidae (fig. 19, 6), Echinocorytidae, Spatangidae (fig. 19, 7), Palaeostomidae, and Pourtalesiidae; Jurassic to Recent. Order 8. _Clypeastroida._--Ambulacrals simple or compound, with two pores juxtaposed, dorsal podia respiratory; interambulacrals multi-tuberculate, none resorbed; mouth central with flattened unequal jaws, reduced external gills, and few sphaeridia; anus exocyclic. Families: Fibulariidae, Laganidae, Scutellidae, Clypeastridae; Cretaceous to Recent. _ Cambrian | | | Ordovician BOTHRIOCIDAROIDA (jaws unknown) | / \ | / \ columns Silurian MELONITOIDEA \ unfixed # CYSTO- \ in ## CIDAR- \ number ### OIDA \ | Devonian ### CIDAROIDA | ### # | ### # | Carboniferous ### ##\ _| ## ## \ ## ## \ _ Permian # ## \ | ## \ | Trias ## DIADEMOIDA columns