AMBULACEA OP THE EECENT DIADEMATIDiE. 99 



tissue. The coarsely laminated condition of the plates is not 

 found far inwards, for they are comparatively solid centrally ; 

 but it appears that the primitive state of these, and indeed of all 

 the plates of the test, was finely laminate. 



The Sukiral Edges between the Compound Plates of the Ambu- 

 lacra. — No special structure is seen, and the edges of the plates 

 are nearly solid looking, the trace of lamination being absent or 

 very slight. But there is some overlapping of the outer surface 

 of one plate over the corresponding surface of the next, and espe- 

 cially where there is a minute tubercle close to the outer edge of 

 the suture. The base of the tubercle overlaps a planed-off sur- 

 face belonging to the next plate. Moreover, when there are 

 large granules or small tubercles on the successive plates, there 

 is a corresponding overlap. 



The Edges of the Median or Vertical Sutures of the Ambulacra. 

 — The small tubercle at the point of each compound plate at the 

 median line plays an important part in this zigzag of sutures. 



As the point of one plate is received into the re-entering angle 

 of the junction of two successive plates, and the small tubercle 

 is on the point, so there is beneath it a deeply cut-away or con- 

 cave surface which corresponds to a projection on the opposed 

 plate. Over this slight projection is, of course, the overlapping 

 tubercle-base, and usually there is a little bevelling there of the 

 angle of the plates of the opposite zone. The faces of the edges 

 of the median sutures are finely laminated, but not so coarsely as 

 those of the sutures between the ambulacra and the interradia. 



The Median Sutures of the Interradia. — These are best examined 

 from within the test. The zigzag, or the median or vertical line 

 of suture of an interradium, is very distinct within the test. 

 But the edges of the plates at the median suture are not in a 

 straight line converging at angles, and they are evidently not 

 united along planes perpendicular to the surface of the test. The 

 zigzag is in slight curves, and the adoral edge of the coronal plates 

 at the median suture is curved adorally, and underlaps (in the 

 proper position of the test) a corresponding depression on the edge 

 of the aboral part of the suture of the actinally placed plate. 



This underlap is very decided in some places, and when the 

 plates are separated, the lamellar expansion of the adoral part 

 of the suture becomes conspicuous. This lamella is a prolonga- 

 tion of the inner layer of the test. 



The opposed edges of both the adoral and aboral sutures are 

 marked by well-developed, distinct, and more or less numerous. 



