1. [CREUSIA] SPINULOSA. Pl. [13], fig. [6 a]-[6 h]: Pl. [14], [6 i]-[6 u], [6 U].
CREUSIA SPINULOSA. Leach (!). Encyclop. Brit. Suppl., vol. 3, Pl. 57, 1824.
CREUSIA SPINULEUSE. De Blainville. Dict. Sc. Nat., Pl. 116, fig. 6.
CREUSIA GREGARIA. G. B. Sowerby (!). Genera of Recent and Fossil Shells, No. 18, Sept. 1823.
---- GRANDIS. Chenu. Illust. Conch. Tab. 1, fig. 2, sed non fig. 2 a and b.
Hab.—Philippine Archipelago, China, Singapore, Java, Red Sea, West Indies; imbedded in various corals; Mus. Brit., Cuming, Stutchbury, Dunker, &c.
General Appearance.—The shell is oval, generally flat, sometimes conical, with narrow and approximate ridges radiating from the orifice (fig. [6 a]). The ridges, however, are sometimes distant from each other, and considerably prominent, projecting round the basal border. The orifice is either neatly diamond-shaped or oval. The four compartments are quite distinct; the radii are generally white, of considerable width, and with their summits not oblique. The colour is either white, or pale pinkish-purple; but in var. 11, bright pink. Even in the white specimens, when well preserved, the sheath is generally, but not always, either pale or dark purple. The largest specimen which I have seen, from the West Indies, was above half an inch in diameter; but from .3 to .4 of an inch is the more usual full size. I believe that the size, as well as the great variability of the present species, is partly determined by the rate of growth of the various zoophytes in which the specimens are imbedded, for the shell has to keep on nearly a level with the surface of the coral.
Structure of Shell and Basis.—The walls are internally ribbed; the ribs being usually prominent, sometimes to such a degree as to deserve to be called plates. The outer lamina is of variable thickness, and the prominence of the internal ribs appears in considerable part to depend on the extent to which the outer lamina has been thickened from within. In many specimens, instead of the interspaces between these internal ribs or longitudinal septa being solidly filled up, separate and successive laminæ have been deposited, by which the shell is rendered cancellated or porose; but the pores are very irregular; and sometimes they form two or three irregular rows one behind the other: this structure seems eminently variable. The edges of the radii are formed by crenated, and occasionally branched, septa. That part of the alæ, which is added during the diametric growth of the shell, is very thin. The lower edge of the sheath seems always to be free. The shelly layer, forming the basis, which is deeply cup-formed, is thin, more or less finely furrowed in radiating lines, and not permeated by pores.
The opercular valves will be best described under the following eleven varieties.