THE AMBULACRA OF FOSSIL ECHINOIDEA. 4:25 



spending plate of a modern Diadema. The middle plate is lowest at 

 the centre of the tubercular area, for it is nipped in by the curved edge 

 of the first plate and also bj^ an aboral curvature of the third plate. 

 But further towards the median line the middle plate, after carrying 

 the mamelon, expands, and is in relation with much of the boss 

 and the greater part of the compound plate. The third i^late has 

 its aboral edge curved apically, and it is a low primary, for the 

 suture between it and the middle plate reaches the median suture 

 just abactinally to the adoral and inner angle of the compound plate. 



The resemblance of these plates to those of the typical and recent 

 Diademata is exact. 



Perhaps the most striking species of Hemipedina is H. tuhercidosa, 

 "Wright (op. ciL p. 164), on account of its resemblance to a Hemi- 

 cidaris without crenulation, and with an unusual number of small 

 secondary tubercles in the interradia. It is a beautiful form, and is 

 even more Cida^^is-looking than Hemicidaris. The ambulacral 

 plates, however, do not always remain as simple primaries; for 

 towards the ambitus, where the tubercles increase in size rather 

 suddenly, there are three pairs of pores evidently in relation to as 

 many plates which have combined to produce a geometrically shaped 

 compound plate (fig. 4). The triple pairs may arch very slightb 



Fig. 4 (see p. 451). 



around or be straight at the edge of a great tubercle, which nearly 

 covers the entire plate. The peripodia, which are only slightly 

 oblique and broadly elliptical in shape, are not so crowded as they 

 are in Hemicidaris ; but they impinge upon the outer flank of the 

 tubercle, and in some specimens their relation to the plates which 

 their pores perforate can be appreciated. 



Taking the first tubercle above the ambitus, it will be noticed to 

 be situated apically to a decidedly large one, and to be separated 

 from it by more space than exists between the other tubercles placed 

 in succession towards the peristome. The three peripodia are in a 

 slight arc, and the most adoral is slightly nearer the tubercle than 

 the others. The tubercle is a broad, low cone, with a well-developed 

 mamelon surrounded by a decided groove. Careful observation 

 proves that the adoral pair of pores has the adoral pore on a line 

 with a transverse suture which separates the combination to which 

 this poriferous plate belongs from the next plate in actinal suc- 

 cession. And on the adoral flank of the tubercle, and nearer the 

 base than the groove around the mamelon, is a line which can be 

 traced from the adoral pore o£ the peripodium which is the middle 

 one of the triplet, over the slope of the boss to the median line of 

 the ambulacrum. 



