Bathyceramaster

Bathyceramaster nov. gen.

Etymology. The genus name is derived from the Greek bathys for depth, signifying the deep depth occurrence of these species and the genus Ceramaster or “clay star”, with which this genus shares a superficial resemblance.

Diagnosis. Abactinal plates tabulate, low to moderate in height with weakly developed fasciolar grooves. Stellate plates on bases of abactinal plates absent. Marginal plates, 70–80 per interradius, differing in morphology and facing laterally with minimal to no abactinal facing surface. Abactinal, marginal body wall thickness thin, paper-like. Abactinal, marginal, actinal surfaces covered by polygonal to round granules. Body strongly stellate (R/ r=1.8–4.0)

Comments. Mediaster elegans, as described by Ludwig (1905) (Figs 2 A–G) lacks the key diagnostic character of Mediaster, the radiating spokes present at the base of each abactinal plate. Fisher (1911: 197) was the first to note that these radiating plates were absent from Mediaster elegans, concluding that “there is no advantage in placing it in the present group,” but took no subsequent action to clarify the misplaced species.

The newly established Bathyceramaster is distinguished from Mediaster based on the absence of radiating ossicles on the base of the abactinal plates (Fig. 2 C), which are present in Mediaster, Rosaster and Nectria (e.g., Fig. 6 A, inset). Given its derivation, Bathyceramaster draws immediate comparison with Ceramaster with which it is superficially similar. Bathyceramaster displays a strongly stellate body form (R:r>1.8, but mostly>3.0 for adults), possesses lateral-facing, marginal plates, with each plate displaying a quadrate outline with width approximately the same as length, relatively shallow fasciolar grooves, weakly developed (very low) abactinal plates and a relatively thin body wall. The type species of Ceramaster, C. granularis and other species similar to it (e.g., C. patagonicus), display a more pentagonal body form (R/r= 1.0–1.6) with very wide superomarginal plates (width is sometimes twice the length), displaying a strongly expressed abactinal-facing surface, and with welldeveloped fasciolar grooves, more elongate tabulae and relatively thick abactinal plates.

Occurrence is also a consideration. Known species of Bathyceramaster also occur at lower bathyal to abyssal depths (roughly 900–4000 m) whereas most species of Ceramaster occur in a shallower range (0–2186 m) (Clark and Downey 1992, Clark 1993).

Type species. Mediaster elegans Ludwig 1905