Order Cidaroidea Duncan
Family Cidaridae Gray
Dicyclocidaris n.g.

Cidarids of medium size, in which the larger interambulacral plates possess two concentric rings of scrobicular tubercles; the outer ring demarcating the boundary of the scrobicule, and the inner ring well within the margin, separated by a greater or lesser interval from the outer. Ambulaeral border of the interambulacral plates denticulate. Tubereles perforate, strongly crenulate.

D. denticulata n.sp.

Dimensions: horizontal diameter ca. 20 mm., height ca. 12·5 mm. (estimated from cotype, a broken internal mould). Probably 7 or 8 interambulacral plates to each vertical series.

The holotype constitutes an external mould of four interambulacral plates which seem to comprise the proximal half of a natural series. The description and photographs (Figs. 1 and 2) are taken from positive casts made from the moulds, Fig. 1 being part of the holotype specimen.

Scrobicules shallow, circular on the larger plates, transversely oval on the smaller proximal ones. Scrobicules of the proximal plates confluent, reaching to both the proximal and distal borders of the plates; on the larger plates, no doubt from the ambital region, the scrobicules reach only to the proximal border of each plate, where they are slightly truncated. Tubercles large, prominent, standing well above the general level of the scrobicule, each with about 12 crenulations. Scrobicular tubercles of the outer ring numbering ca. 16 on an ambital plate, and the inner ring comprises about the same number. The inner ring may be separated from the outer ring by an interval only as wide as one serobicular tubercle, as in the holotype (Fig. 1), or the inner ring may lie midway between the outer ring and the base of the parapet, as in a cotype (Fig. 2). Both scrobicular rings are interrupted by the truncation of the scrobicule at the proximal margin. Tuberculation outside the scrobicule not well preserved, but apparently similar to the scrobicular tubercles in form and size, but irregularly

arranged and less distinctly differentiated. The ambulacral border of each interambulacral plate bears on its inner surface 6 or 7 denticulations (Fig. 3, from an internal natural mould).

Ambulacra, peristome, periproct, apex and spines unknown.

Locality: Wai-iti Survey District, Nelson, east side of Castle Hill. Grid reference S20/427097. G.S.4560.

Horizon: Carnic (upper Triassic).

Type and Cotypes: In the collection of the Geological Survey Office, Wellington.

Fig. 3—D. denticulata, another cotype, an internal natural mould of part of an interamb, showing impressions (Dent.) left by the denticulate edge of the ambulacral margins of the plates. The ambulacral margins of the left column of plates have been eroded away.