Prof. S. Loven on the Structure of the EcMnoidea. 293 



All primaiy plates, even the half ones, are originally in their 

 lirst foundation entire plates ; that is to say, they reach from 

 the interradium to the median suture of the ambulacrum. 

 Subsequently, whilst the whole of the complex of primary 

 plates which forms large plates increases in breadth, and even 

 before it is completed by the last primary plates, the inter- 

 mediate ones fall off in their growth ; and whilst they retain 

 their position in the boundary of the ambulacrum towards the 

 interradium, their narrowed ends become more remote from 

 the median suture of the former. The first formed of these 

 intermediate plates is the smallest of all, the later ones become 

 gradually larger ; and thus it hajjpens that whole groups of 

 intermediate primary plates acquire forms of a triangular figure, 

 the apex of which is formed near the middle of the large plate 

 by the projecting end of the last alone. By all this it is also 

 clear that these intermediate plates are not of later origin, 

 neither secondary nor inserted, but that they are formed in 

 ordinal sequence with the two outer entire plates. But the 

 latter grow to a much greater degree, so that they directly 

 touch each other where the intermediate plates cease, constitute 

 the greatest part of the area of the large plate, and the whole 

 of its margin towards the median suture. 



The youngest large plates are distinctly longer, in the 

 direction from the vertex towards the peristome, than broad ; 

 but in proportion as each large plate grows, and at the same time 

 is removed from the vertex, it becomes broader in proportion to 

 its length. The greatest jjeriphery of the corona is always 

 so placed that half the number of the plates and something 

 more is ventral — that is to say, situated between it and the 

 peristome, notwithstanding that the distance from it to the 

 peristome is always less than to the vertical rings. Con- 

 sequently during growth a compression from above downwards 

 of the ventral plates takes j)lace, which appears more strongly 

 in proportion to their age, and, in combination with the move- 

 ment which also takes place in each large plate, alters their 

 form in a regular manner and at the same time changes the 

 position of the pores. In the youngest individuals which 

 have been examined, all the tentacular pores (with the excep- 

 tion of the very first interrupted one) are placed near the suture 

 towards the interradium, and those which belong to the same 

 large plate form together a curve with a slight, outwardly 

 convex flexure. These are the primordial pore-arcs. But 

 the tentacular pores begin very soon to move, in order to take 

 up a different position, and finally form other secondary arcs, 

 which remain the same during the animal's whole life, and are 

 SO characteristic that we derive from them the characters of 



