734 
PROFESSOR WYVILLE THOMSON ON THE ECHINOIDEA OF THE 
upwards of 4 millims. in diameter, surrounded by a distinct areolar ridge with miliary 
tubercles, the areolae sometimes tending to become confluent. On the inside of the 
test, owing to the thinness of the shell, the areolae have the appearance of raised bullae 
(Plate LXII. fig. 1, Plate LXIII. figs. 1, 2, 3). The mamelon is large and hollow, with 
an entire porcellanous ring ; the tubercle is rather small and perforated. The general 
arrangement of the plates is the same on the lower as on the upper surface, but it is 
difficult to make out the sutures from the extraordinary size of the scrobiculae. The 
pairs of pores are jostled into single file, and find their way to the edge of the peristome 
as best they can. 
The apical opening (Plate LXIII. fig. 5) is very large and star-shaped, owing to the 
passing down into the interambulacral areas of a triangular prolongation of the mem- 
brane of the periproct, in the centre of which the ovarial duct opens. Over each ovarial 
aperture there is a small crescentic calcareous plate ; and this, with a set of small calca- 
reous granules imbedded in the membrane round the opening, represents the large ovarial 
plate, which is usually developed in this position. The madreporic tubercle, which is 
reniform and of large size, is in the position of the crescentic plate above one of the ovarial 
openings. The ocular plates are quadrate, with the angles rounded. The anal aperture 
is central, and the membrane of the periproct is thickly studded with small round plates, 
most of them bearing a miliary granule and a spine, the interstices between them filled 
up with calcareous granules thickly set. The plates decrease in size towards the anus, 
and immediately round the opening they become lengthened, and have almost the 
character of spicules converging towards it. 
The peristome is mailed with ten double rows of thick calcareous imbricated scales, 
five double rows ambulacral and five interambulacral, all overlapping towards the mouth. 
The ambulacral scales are perforated for the passage of processes from the ambulacral 
canals which pass right up to the edge of the mouth-opening (Plate LXII. fig. 1). 
There are two kinds of spines, one articulated to the primary tubercles, and the other 
to the minute miliary tubercles. The larger spines (Plate LXII. fig. 3) are from 10 to 
15 millims. in length, with the shaft 5 millims. in diameter; they are hollow, consisting 
of a fenestrated calcareous tube with eight to ten projecting ridges, which are here and 
there raised into obliquely ascending spines. Sometimes a spiral arrangement may be 
detected in their projections, as we see in the very small spines of Diadema. The 
proximal end of the spine is enlarged, with a marked ring, and a space for the insertion 
of the large muscular mass which fills the deep alveola. The acetabulum is shallow, with a 
smooth margin and a central pit for the “ round ligament.” The distal end of the spine 
is drawn to a transparent point, like the drawn-out and sealed end of a glass tube (Plate 
LXII. fig. 3). The smaller spines are excessively fine hair-like fenestrated calcareous 
tubes. The spines are most numerous towards the edge and on the oral surface of the 
body, the small spines, mixed with pedicellariae, clothing the scales of the peristomial 
membrane close up to the mouth. It is difficult to imagine the object of the peculiar 
arrangement on the oral portion of the corona ; the size of the areolae and of the masses 
