ECHINUS. 299 
it is sub-conical, in others depressed. The region imme- 
diately surrounding the peristome (Pl. L., fig. 7, per.) is 
somewhat flattened. The outline seen when the test is 
viewed from either pole is the ambitus. The spines, with 
which the test is thickly covered, are usually whitish at 
their bases, more or less of their distal ends being tinged 
with pale violet. In what appears to be a rare colour — 
variation, the spines are white, with an ill-defined blackish 
band about the middle of their length. In addition to the 
spines, the test bears a large number of pedicellarize 
(elapiie). Lhe mouth (Pl. I., fig. 7, mo.) occupies the 
centre of the broader and shghtly flattened pole—hence 
the oral pole—of the test, and from it may be seen pro- 
jecting five calcareous teeth (to.). It is surrounded by the 
membranous peristome. The anus (Pl. 1., fig. 1, an.) is 
situated at the opposite pole of the test, a little to one 
side of the centre of a smaller but similarly membranous 
periproct (pp.). 
The test is composed of twenty longitudinal rows of 
calcareous plates (Pl. I., figs. 8 and 4), and ten plates, 
which encircle the periproct, at the aboral pole. The 
latter form the so-called apical system (Pl. L, fig. 1). All 
the plates are firmly united by suture, and growth takes 
place at their edges, there being a thin and continuous 
layer of uncalcified connective tissue between the apposed 
margins. Of the twenty longitudinal rows of plates five 
double rows form the ambulacra or radu; the remaining 
five double rows alternate with them, and form the inter- 
ambulacra or interradi. The structure and relations of 
the several kinds of plates will be readily understood by 
reference to figs. 1, 3, and 4, Pl. I. Figure 3 represents 
two plates taken from one row—the right, facing the 
observer—of an ambulacruim, in the region of the ambitus. 
The triangular ends on the left were apposed to exactly 
