250 ROBERT TRACY JACKSON ON ECHINI. 



Order ECHINOCYSTOIDA nom. nov. 



Cystocidaridae Zittel, 1879, p. 480; A. Agassiz, 1881, p. 81. 



Cysiocidaroida Duncan, 1889a, p. 20; Jackson, 1896, p. 242; Gregory, 1900, p. 300. 



Cysticaroidea Tornquist, 1897, p. 730. 



Professor Zittel based his name of the order on the name Cystocidaris which he gave to 

 replace the genus Echinocystites of Wyville Thomson. As Cystocidaris is a synonym and 

 cannot stand, neither, by the rules of nomenclature, can the name of the order based on it; 

 I therefore substitute a new name for this order. 



Test spheroidal or flattened, apparently irregular, with the anus in an interambulacral area. 

 Ambulacra narrow, with two or four columns of low plates in an area. Interambulacra wide, 

 with numerous, eight or more, columns of very thin, rather irregular polygonal plates in an 

 area. Lantern well developed, typically echinoid in character. The known specimens of the 

 two families and genera of this order are imperfectly preserved, therefore are incompletely 

 known; further knowledge is much to be desired. 



Family PALAEODISCIDAE Gregory. 

 Palaeodiscidae Gregory, 1897, p. 133; Lambert and Thiery, 1910, p. 117. 



Test depressed, nearly or quite circular. Gregory says pentagonal, but I think this is 

 hardly correct. Ambulacra narrow, composed of two columns of low plates in each area, with 

 pores throughout the areas, not imperforate ventrally, as Gregory says. Anus eccentric in 

 an interambulacrum at a considerable distance from the apex and quite near the mid-zone, as 

 figured by Spencer (1904, Plate 1, fig. 1). Earlier Gregory (1897) said that the anus is central, 

 but he did not figure it. I have not seen a specimen showing this structure. A typical echinoid 

 lantern exists. There is only one genus and species. 



PALAEODISCUS Salter. 



Palaeodiscus Salter, 1857, p. 332; Zittel, 1879, p. 453; Duncan, 1889a, p. 6; Gregory, 1900, p. 301; Lam- 

 bert and Thiery, 1910, p. 117. 



With characters of the family. The type and only recognized species is P. ferox Salter, 

 of the Silurian of England. Palaeodiscus gothicus is considered under Incertae Sedis. 



*Palaeodisaus ferox Salter. 

 Plate 18, figs. 1-5. 



Palaeodiscus ferox Salter, 1857, p. 332, Plate 9, fig. 6; Wyville Thomson, 1861, p. 13, Plate 4, figs. 6, 7; 

 Neumayr, 1881, p. 155, Plate 1, fig. 8; Gregory, 1897, p. 129, text-figs. 4, 5; Plate 7, figs. 5a-5b; 

 Sollas, 1899, p. 701, text-figs. 6-11; 1899a, p. 277; Spencer, 1904, p. 31, text-figs. 1-8, Plate 1, fig. 1; 

 Lambert and Thiery, 1910, p. 117. 



