The Echinoid Directory

Palaeodiscus Salter, 1857, p. 332

[?=Lepidechinoides Olsson, 1912, p. 442, type species L. ithacensis Olsson; ?= Cavanechinus Browne, 1967, p. 158, type species Cavanechinus warreni Browne, 1967 ]

Diagnostic Features	Test composed of very thin, imbricate plates. Apical disc unknown; relatively small. Ambulacra narrow, straight, plating biserial, strongly imbricate with large flange-like extension beneath interambulacral zones. Pore-pairs uniserial, adradial; fully enclosed in plate. Perradial flange present internally, fully enclosing radial water vessel. Interambulacra composed of up to 8 semi-regular columns of plates; plates rhomboidal to semicircular. Plates with numerous small tubercles. Peristome small, with ambulacral plates. Lantern present; tooth U-shaped in cross-section, not constructed from large alternating biseries of elements. Rotulae and epiphyses developed. Spines short and simple, forming a rather dense felt.
Distribution	Upper Silurian (Ludlow), England; possibly Devonian USA, Australia.
Name gender	masculine
Type	Palaeodiscus ferox Salter, 1857, p. 332, by original designation. Neotype (selected by Hawkins & Hampton, 1927, p. 588) British Geological Survey, Geological Survey Museum 47577.
Species Included	P. ferox Salter, 1857; Upper Silurian, Engalnd. ?P. warreni (Browne, 1967); Middle Devonian, Australia. ?P. ithacensis (Olsson, 1912); Devonian, USA. ?P. hunti (Cooper, 1931); Devonian, USA. ?P. whitnalli (Cooper, 1931); Devonian, USA.
Classification and/or Status	Stem group Echinoidea; Lepidocentridae. Presumed paraphyletic, by exclusion of Lepidechinus.
Remarks	This species is easily distinguished from Echinocystites, the other echinoid occurring in the same beds, by having biserial ambulacral plating, not quadriserial plating. Aptilechinus has narrower interambulacra with plates only bearing a single small tubercle. Lepidechinus has somewhat thicker interambulacral plates that are very definitely arranged into en chevron rows. Hawkins, H. L. & Hampton, S. M. 1927. The occurrence, structure, and affinities of Echinocystis and Palaeodiscus. Quarterly Journal of the Geological Society London 83, 574-603, pls 44-46. Salter, J. W. 1857. Annals and Magazione of Natural History, series 2 20, p. 332.