Fig. 6.

Tuberculate layer surface. a General view of the transition from the annular layer surface (AL) to the tuberculate layer surface (TL). b Details of the transition between the AL and TL. c, d Details of tubercles in the transition zone between AL and TL surfaces; (note the irregular shape of tubercles that nearly fuse and that have different morphology from those occurring further along the spicule); e Typical, well-developed tubercles further from the annular surface (toward the apex of the spicule). f Typical tubercles in early stage of formation (young) on the same surface but located more apically