Sibogaster Doderlein 1924

Sibogaster Döderlein, 1924

Döderlein, 1924: 63; A.M. Clark 1993: 283 (checklist only)

Type species. S. digitatus (by monotypy)

Taxonomic comments. Sibogaster has not been collected since its original description by Döderlein (1924). The second species, S. nieseni, n. sp. described below expands the concept of Sibogaster based on greater character variation, including further differences in glassy bosses among species.

The type species, S. digitatus possesses glassy bosses embedded on flattened, abutted abactinal plates, a character which is shared by Plinthaster and Astroceramus. Both species of Sibogaster presented herein possess bare abactinal plates with no surficial granules or other accessories. Granules on superomarginal plates in Sibogaster are either weakly expressed or absent. Superomarginals in both species are elongate (L>W) interradially.

Sibogaster shares a strong affinity with Pergamaster particularly Pergamaster incertus (Bell, 1908). Both S. nieseni and P. incertus are devoid of abactinal granules, have similar marginal plate arrangement and morphology and similar overall body shape. They differ in that granule and abactinal plate shapes and size are significantly different in size for similar-sized individuals and in that Pergamaster displays significantly larger and thicker subambulacral and furrow spines are significantly larger and thicker.

Diagnosis. Abactinal plates smooth, bare (no accessories on plate surface-although embedded glassy bosses present in S. digitatus), weakly convex, round to polygonal in outline. Marginal plates elongate, mostly smooth, bare (S. digitatus has fewer than 6 granules on one or two interradial superomarginals). Inferomarginals adjacent to actinal surface covered with variably abundant granules. Actinal plates covered by granules. Adambulacral plates rectangular in shape. Furrow spines numerous, six to 15. Subambulacrals. thickened relative to furrow spines, remaining granulation identical to others on actinal surface.