SPATANOOIDA  Cretaceous CLYPEASTROIDA 1

Tertiary Recent L--Jaws lofty jaws flat—ryduced — lost ~— abranchiate —JL—branchiate wt—lipobranchiate—' L— no sphaeridia —a' .sphaeridia endocyclic exocyclic 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 . zoologie concrete, iii., 1878–1887i Y . (Paris, 1904) ; A . Lang, Text-Book of Comparative Anatomy, transl., part ii . (London, 1896); Ludwig and Hamann, " Echinodermen," in Bronn's Klassen and Ordnungen des Tierreichs (Leipzig, 1889), in progress; M . Neumayr, Die Stdmme des Tierreiches (Wien, 1889); P . B. and C . F .

Sarasin, Uber die Anatomie der Echinothuriden and die Phylogenie der Echinodermen," Ergebnisse naturw . Forsch. auf Ceylon, Bd. i Heft 3 (Wiesbaden, 1888) ; R . Semon, " Die Homologien innerhalb des Echinodermenstammes," Morph . .Tahrb . (1889); W . P . Sladen, " Homologies of the Primary Larval Plates in the Test of Brachiate Echinoderms," Quart . Journ . Micr . Sci., 1884; K . A. v . Zittel, Handbuch der .

. . Palfiozoologie, i. pp . 308-560 (Munchen, 1879); also Grundsuge, translated and revised by C . R .