Description of a new Species of Holaster

Abstract

Holaster, rotundus,sp. nov. (Plate XII.) Description .—Test large, rather thick, tumid, circular, or broadly ovate, but generally as broad as long. Upper surface elevated and sloping evenly all round towards the ambitus, Under surface flattened, but rounded at the border. Mouth opening in a slight depression near the anterior border; anal opening above the base on the posterior border, with a small depression or hollow below it. The dimensions of full-grown specimens are two and a half inches in breadth or transverse diameter and the same in length; height about two inches. The ambulacral summit is central; the ambulacra, which are straight, lanceolate, and apetaloid, radiate from it. The anteal sulcus is feebly developed, the depression in front of the mouth being continued for a short distance along the ambulacral area, but hardly far enough to break the circular outline when the test is viewed from the summit. Both the antero- and postero-lateral ambulacra are straight and nearly equal; the poriferous zones are similar and narrow; the pores are clearly visible from summit to base, about 50 being visible in each avenue of the anterior pair, but not quite so many on the posterior pair. The apices of the three anterior pairs converge at a little distance from the posterior pair, just as in Holaster subglobosus . The composition of the apical disk appears to be as usual in the genus Holaster , and the madreporiform body is distinct in the specimens examined. The interambulacral areas are formed of large wide plates, those