Fig. 9.

Structure of the annular layer (AL) and its relation to the underlying tuberculate layer (TL) and plain glassy silica (PG). a General view of all 3 zones in the spicule: PG, TL and AL. b, c Contact between AL and underlying TL (note that tubercles fit perfectly into depressions in the lower surface of the AL), it resembles tenon and mortise or dovetail joints. d Granular characters of silica in the AL (note layering). e, f Details of the AL lower surface showing complex regular depressions into which fit tubercles from the underlying layer