## # \ 20 Jurassic ## ## \ HOLECTYPOIDA | ## ### / # \ SPATANGOIDEA | Cretacious ## ### CLYPEASTROIDEA # \ # | ## ### # ## ## | Tertiary ## ### ## ## ### | ## ### ### # ### | Recent ## ### ### # #### _| |----------jaws lofty--------| jaws flat-reduced ---- lost |------abranchiate-----| |-----branchiate-----| |-lipobranchiate-| |----no sphaeridia-----| |--------------sphaeridia---------------| |-------endocyclic---------| |------------exocyclic--------------| [Illustration: FIG. 20.] The probable relationship of these orders is shown in the annexed table. Here the Cystocidaroida occupy an isolated position. It is, however, quite possible that _Echinocystis_ may some day be referred to the Cidaroida, and _Palaeodiscus_ to the Melonitoida. This would leave the Echinoid scheme remarkably simple, with the Melonitoida and Cidaroida as divergent branches from an ancestor like _Bothriocidaris_; but while the former branch soon decayed, the latter continues to flourish at the present day. To take the Echinoidea now living, and to divide them into Endocyclica and Exocyclica, Branchiate and Abranchiate, Gnathostomata and Atelostomata, is easy and convenient; or again to distinguish as Palechinoidea those pre-Jurassic genera which do not conform to the fixed type of twenty vertical columns found in the later Euechinoidea, is to express an interesting fact; but all such divisions obscure the true relationships, and the corresponding terms should be recognized as descriptive rather than classificatory.