ANATOMY OF TIIE TEST OF DISCOIDEA CYLINDRICA. 
53 
upper one curves aborally to reach the ahoral suture of the com- 
pound plate, and the adoral demi-plate has its ahoral suture curved 
so as to reach the adoral suture of the compound plate. 
The next compound plate (2) is formed in the same manner as 
that just described ; but the demi-plates are smaller, and are per- 
forated by pairs of pores which belong to the inner set ; on the 
other hand, the primary placed between the demi-plates has a 
pair of pores of the outer set, and this is the reverse of what 
occurs in the plate above. Plate 3 has only two components, 
and the abactinal plate is a low broad demi-plate perforated by a 
pair of pores of the outer set ; the ac-tinal plate is a low primary, 
and its pair of pores belongs to the inner set. 
Plate 4 is formed like plate 2, but the demi-plates are larger; 
moreover, the pairs of pores of the upper demi-plate are of the 
inner set, like the pair seen in the demi-plate placed immediately 
abactinally in the compound plate above ; but they are placed 
nearer the interradium than the pair above, and the result is to 
give a triple series of pairs of pores with the pair of the primary 
of plate 4. This triple appearance is rare in Discoidea. It will 
be observed that the composition of the four compound plates, 
omitting the biserial condition, is suggestive of that seen in the 
genus Coelopleurus *. 
At some little distance from the peristome the alternation of 
outer and inner pairs of pores continues, and the compound 
plates are rarely formed of more than two plates. The primary 
of the compound plate is low aud broad, and is placed abactinally, 
whilst the very small demi-plate is found at the actiual aud 
outer part of the plate. When a tubercle 
is present, the primary becomes high. The 
primary has a pair of the outer series of pores, 
and the demi-plate a pair of inner series 
(fig. 6). 
In half-grown specimens a decided change 
occurs in the pairs of pores at a little distance from the peri- 
stome, and they become larger, wider apart, and the arrangement 
is nearly in simple series (fig. 7). The pores of each 
pair are very oblique and almost vertical; and the 
greater number belong to primaries which are higher 
than those nearer the ambitus. A few are in demi- 
plates. In older specimens the simple series close 
to the peristome is much shorter, and indeed barely 
* Duncan and Sladen, Journ. Linn. Soc., Zool. vol. xix. 1885, pi. i. 
Fig. 7. 
Pig. 6. 
