The Echinoid Directory

Actinocidaris Mortensen, 1928, p. 73

Diagnostic Features
  • Test rather tall; sides arched, top flat; thick.
  • Apical disc about half test diameter; ocular plates just exsert or with one plate just insert. Genital plate 2 not larger than other genital plates. Periproctal plating extensive.
  • Interambulacra with between 6 and 9 plates in a series. Primary tubercles perforate and non-crenulate; areoles very shallow. Scrobicular tubercles differentiated and separate, except immediately beside the peristome where they may be confluent. Extrascrobicular zones well developed and with coarse and dense tuberculation.
  • Ambulacra sinuate and moderately broad. Pore-pairs horizontal in orientation and well developed; the two pores separated and conjugate, though with the interporal partition raised. Each plate with a marginal primary and one or more slightly smaller inner secondary tubercles.
  • Peristome smaller than apical disc with ambulacral plates uniserially arranged. Interradial plates confined to outer part of peristomial membrane.
  • Primary spines stout and weakly fusiform, tapering to a blunt point. Shaft ornamented by fine granules more or less regularly arranged, but not forming distinct ribs. Cortical hairs well developed.
  • Secondary spines weakly flattened; not strongly adpressed.
Recent, Hawaii.
Name gender feminine
Phyllacanthus thomasi Agassiz & Clark, 1907, p. 15, by original designation.
Species Included
  • Mortensen included only the type species.
Classification and/or Status

Cidaroida, Cidaridae, Cidarinae.

Monotypic; subjective junior synonym of Prionocidaris.


Distinguished from Eucidaris by is more fusiform spines with a more spinose ornament, and by having clearly conjugate pore-pairs. Phyllacanthus differs in having an enlarged madreporite and biserial pore-pairs on the outer part of the peristome. The type species has rather stouter and more fusiform spines than is typical of Prionocidaris, but little else distinguishes the two.

Agassiz, A. & Clark, H. L. 1907. Hawaiian and other Pacific Echini. Memoirs of the Museum of Comparative Zoology at Harvard University 34(1), 1-42, pls 1-42

Mortensen, T. 1928. A monograph of the Echinoidea. 1, Cidaroidea. C. A. Reitzel, Copenhagen.