ANATOMY OB' TIIE TEST OE DISCOIDEA CVLINDRICA. 
51 
Fie. 1. 
(^® qQ 0 Q 0^ _ 
they are very obliquely placed in reference to the transverse 
sutures of the ambulacral plates. The outer pore is aboral, and 
is very close to the edge of its plate andtheinterradium ; and the 
inner pore is adoral, and either close above 
the line of suture or on it; and in the first 
instance a narrow linear prolongation of the 
pore may occasionally be seen passing down to 
the suture between the plate and the next in 
ad oral succession (figs.landl«). (B.M.3S742.) 
The number of these primaries is consider- 
able ; and they almost reach the ambitus in 
specimens which are not adult. They are followed by, or alter- 
nate with, compound plates composed of a primary and a low, 
Fig. la. 
Fig. 2. 
short demi-plate (fig. 2), or a demi-plate 
may be intercalated between two primaries, 
the three not forming a compound plate 
(fig. 2 a). (B.M. 723.) The demi-plate in 
this instance was a primary which has been 
crowded-out by the growth-pressure of the 
primary above and below ; but in the other 
instance (fig. 2) the demi-plate has been so 
pressed upon that it has been fused, as it 
were, with the primary. 
Both in the specimen marked 723 in the British Museum and 
in one in our possession the primaries are followed, at the ambitus 
or just above it, by taller compound plates, each of which consists 
of a large primary placed abactiually, and a low, 
broad, triangular demi-plate situated actinaily. 
The two plates are united by very delicate 
sutures and form a geometrical compound plate 
(fig. 3). (See also B.M. ISO.) 
Sometimes, at the ambitus, there is a third 
plate in a compound plate ; and the arrange- 
ment seen is very unusual in the Echinidae. Eor : — (1) The pairs 
of pores are very slightly out of the straight vertical line. (2) 
The upper plate is a large primary which occupies the whole of 
Fig. 3. 
very narrow, and have one pair of small round holes opposite each ambulacral 
plate.” The pores are of course in ambulacral plates. On the same page it is 
stated, “ near the mouth-opening two rows [of pairs of pores] go to each plate.” 
This is incorrect ; for there is but one pair to a plate. 
