ANATOMY OF THE TEST OF DISCOIDEA CrLINDRICA. 



51 



Fig. la. 



Fig. 2. 



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 -p-g. ^ 



the line of suture or on it; and in the first 



instance a narrow linear prolongation of the (^^q ^ 0^ 

 pore may occasionally be seen passing down to 

 the suture between the plate and the next in 

 adoralsuccession (figs. 1 and 1 «). (B.M. 38742.) 



The number of these jDrimaries 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, 

 short demi-plate (fig. 2), or a demi-plate 

 may be intercalated between two primaries, 

 the three not forming a compound plate 

 (fig. 2«). (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 comjoound plates, each of which consists 

 of a large primary placed abactinally, and a low, 

 broad, triangular demi-plate situated actinally. 

 The two plates are united by very delicate 

 sutures and form a geometrical compound plate 

 (fig. 3). (See also B.M. 180.) 



Sometimes, at the ambitus, there is a third 

 plate in a comj^ound plate ; and the arrange- 

 ment seen is very unusual in the Echinida?. ¥ot : — (1) The pairs 

 of pores are very slightly out of the straiglit vertical line. (2) 

 The upper plate is a large primary which occupies the whole of 



Fig. 2 a. 



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. 



I ) r\c III I \0 



