‘ PORCUPINE ’ DEEP-SEA DEEDGING-EXPEDITIONS. 
733 
body is nearly flat. The interambulacral plates of the apical surface are transversely 
oblong, the alternating double row meeting in the central line of the interambulacral 
area in a serrated suture ; thus the plates of the opposite sides of the area slightly over- 
lap, and all the plates of the interambulacral area slightly overlap one another from the 
circumference of the test towards the apex. The outer edges of the interambulacral 
plates are truncated, and form together nearly a straight line, the whole series over- 
lapping the outer edges of the plates of the adjacent ambulacral areas (Plate LXIII. 
figs. 6 & 7). The plates of the ambulacral areas are thus essentially within those of the 
interambulacral areas. The first four or five interambulacral plates of each area 
towards the apex are separated — in four of the areas to accommodate the outer angle of 
the groups of tubercles which represent the genital plates in this genus, and in the 
fifth to admit the madreporic tubercle (Plate LXIII. fig. 4). 
The surface of the interambulacral plates is smooth, with sparsely scattered primary 
tubercles, a few secondary tubercles, and a few minute miliary grains (Plate LXIII. fig. 5). 
The primary tubercles are one or two on each plate towards the apical end of the area ; 
but they become more numerous, two at least, and often three, on each plate towards 
the circumference. The primary tubercles are perforated. The mamelon is surmounted 
by a distinct porcellanous ring, but is not crenulated. The areola is well marked, 
slightly excavated, and about L5 millim. in diameter. The number of interambulacral 
plates in series, from the circumference of the test to the edge of the periproct, is 
about ten. 
The ambulacral plates of the apical surface are likewise oblong, in double series ; 
meeting in the centre of the ambulacral area in a serrated suture. The outer edges of 
the plates are slightly rounded. The plates overlap from the apex towards the circum- 
ference. The outer edges of the ambulacral plates are entirely covered by the outer 
ends of the plates of the adjacent interambulacral series. Nearly midway between the 
central line of the ambulacral area and the outer edge, two small accessory plates, each 
perforated with a double pore, are intercalated between each contiguous pair of ambu- 
lacral plates ; and another pair of pores, forming the lowest pair of an arc, penetrates 
the substance of the ambulacral plate a little nearer the outer edge. The pores are 
therefore trigeminal (Plate LXIII. figs. 5 & 6). Each ambulacral plate usually bears 
towards the centre a single primary tubercle or two smaller-sized tubercles, or, in some 
cases, only miliary granules. The number of ambulacral plates in series from the edge 
. of the periproct to the circumference is about fifteen. 
At the circumference there is an abrupt change of character. J ust above the edge 
there is a nearly continuous band, running through all the plates, of large miliary tuber- 
cles bearing five spines, which form a kind of marginal fringe ; and beneath that, towards 
the lower surface, an interrupted smooth band with neither tubercles nor granules ; and 
the ambulacral and interambulacral areas then proceed towards the edge of the peri- 
stome. Each interambulacral plate is deeply excavated or impressed with two or three 
disproportionately large and deep areolse, which appear on the outside like deep cups 
