CREUS1A SPINULOSA. 377 



(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 

 laminae 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 alas, 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 fur- 

 rowed in radiating lines, and not permeated by pores. 



The opercular valves will be best described under the following eleven 

 varieties. 



Var. (1), PI. 13, fig. 6#, 6 b, 6 c, 6 d. — Hab. Java, and probably 

 several other districts. — I will first describe a typical sub-variety, by 

 which I mean a sub-variety not presenting any extreme character. The 

 scutum is of a sub-triangular shape, with the basi-tergal corner much 

 rounded off (6 d), and generally but not always hollowed out in a rather 

 remarkable manner. The adductor ridge is considerably prominent, 

 and extends high up, parallel to the, articular ridge, which latter is 

 rounded and prominent, but to a variable degree. Near the rostral angle 

 there is sometimes a small tooth, or only a trace of one, depending 

 beneath the basal margin ; this tooth we shall hereafter see much more 

 developed. The Tergum is about two thirds of the width of the scutum. 

 It is often slightly beaked, but this is more conspicuous in some of the 

 following varieties. The spur is about half the width of the valve, and 

 its basal end is truncated, and nearly parallel to the basal margin of the 

 valve, but the truncated form passes insensibly into a rounded outline. 

 The shell in this variety is generally thick and is not permeated by pores ; 

 the orifice is diamond-shaped. But in another sub-variety the walls of 

 the shell are always, or nearly always, permeated by pores, and the 



