ANATOMY OF THE TEST OF DISCOIDEA CrLINDRICA. 



53 



upper one curves aborally to reacli the aboral suture of the com- 

 pound plate, and the adoral demi-plate has its aboral 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-j)lates 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 actinal plate is a low primary, 

 and its pair of pores belongs to the inner set. 



PJate 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 tlian two plates. The primary 

 of the compound plate is low and broad, and is placed abactinnlly, 

 whilst the very small demi-plate is found at the actinal and 

 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. 



