THE AMBULACRA OF FOSSIL ECmNOIDEA. 439 



this union produced a symmetrical plate towards the median line. 

 The tubercle of plate 36 intrudes on plate 35, the adoral suture of 

 which crosses the boss to reach the median line not far from the 

 apical angle of the compound plate : this suture is curved, with the 

 convexity directed adorally. A few granules are placed between 

 the tubercle and the median line, and upon the apical part of 

 plate 35. 



On comparing plates 35 and 36 with plates 16 and 17, it would 

 appear that the resisting power of the plate 35 was greater than 

 that of plate 16; but this is the first evidence we have of the 

 direction of the aboral plate of a combination. It is important to 

 observe that the poriferous area of plate 35 is higher in measurement 

 than the part close to the median line. 



The tuberculiferous area of plate 36 is much larger than the 

 corresponding poriferous area. The next plate, 37, is a small low 

 primary, with granules, and the first step towards its organic 

 connexion with plate 36 is the symmetrical arrangement of the 

 granules in conformity with their place and trend in the two other 

 plates 35 and 36, which certainly do form a double-pored combi- 

 nation. 



The structure of the plates immediately aborally to the first large 

 tubercle at the ambitus, counting from the radial plate, is simple as 

 a rule, and differs in a remarkable manner from that of the great 

 tubercle-bearing compound plates. Taking the Amb. III. of the 

 species of Remicidaris, and the zone " a," it will be noticed that the 

 small compound plate placed aborally to the great tubercle, consists 

 of three low primary plates united to form a symmetrical com- 

 bination (fig. 13)*. The tubercle which is placed on this compound 



Pig. 13 (see p. 452). 



plate is small, and its boss is crossed, from the poriferous area to the 

 median line, by the sutures of the aboral and adoral plates (37 & 39) 

 where they are in contact with the edges of the central plate, which 

 usually has the mamelon upon it (38). The line of the sutures is almost 

 transverse in most instances ; but it may happen that the adoral 

 plate of the combination is so jammed by the huge tubercle of 

 the plate immediately on the actinal side, that it becomes a demi- 

 plate, because it cannot reach the median line (fig. 14). 



The following is the construction of a large tubercle-bearing plate 

 at the ambitus (figs. 13, 14). 



* Specimen 24122, British Museum. 



2h2 



