Bd. VI: 4) 



THE ECHINOIDEA. 



S3 



The test is regularl)- rounded, sometimes with a slight indication of being penta- 

 gonal; it is low, somewhat flattened above, the height only half the horizontal dia- 

 meter; the actinal side nearly flat, the edge of the peristome scarcely incurved. 



The ambulacra are rather broad, nearly as broad as the interambulacra, as seen 

 from the measurements. The primary tubercles, which — as is the case in all poly- 

 porous forms — occur on all the compound plates, make two regular series, gene- 

 rally considerably smaller than those of the interambulacra. The interporiferous zone 

 is comparatively narrow, in consequence of which the larger secondary tubercles 

 inside the primaries form only a single median series on the actinal side (PL VI 

 Fig. 2); above the ambitus, where the plates are wider, the.se tubercles are arranged 

 so as to form two alternate series. This is, however, not always the case. In the 

 largest specimen in hand, as well as in another specimen of ji mm. they are arranged 

 in distinct double series, except at the upper and lower end. In the smallest spe- 

 cimen in hand (the one of 42 mm. in the above measurements) they form onl}' a 

 single series in the whole ambulacrum. This larger secondary tubercle is placed in 

 the inner corner of the ambulacral plate, opposite to or a little higher than the 

 primary tubercle. The rest of the plate inside the primary tubercle is covered by 

 more or less numerous miliary tubercles, one at the upper edge, almost midway 

 between the median line and the innermost pore, being slightly larger than the others. 

 The pore areas are rather broad; the pores, generally 7 — 9 in each series, are arranged, 

 on the actinal side, in nearly horizontal rows, slightly S-shaped. Above the ambitus 

 the arcs are more erect, the outer 3 — 5 pores (sometimes even 6) forming a straight 

 vertical series close to the outer edge, the remaining 2 — 4 upper pores alone being 

 so placed as to form a curve towards the middle line of the area. The 2 — 3 pore- 

 arcs nearest the peristome have only 3 pairs of pores; the third — fourth has four pairs, 

 the 5th — 6th five pairs etc. The arcs of pores are separated by oblique rows of 

 3 — 4 secondary tubercles. On the actinal side there is only one such row of 

 tubercles between each two pore-arcs, while on the abactinal side there are some 

 smaller tubercles above this row, more or less distinctly serially arranged, so that 



