FIGURE 4 in The tube of Ditrupa bartonensis (Annelida, Serpulidae), from the Eocene of southern England: Observations on microstructure and its significance

FIGURE 4. Scanning electron images of the upper structural layer from Ditrupa bartonensis. A)-F) Showing crosssections through ridged crystals. Note that crystals commonly have triangular or wedge-shaped cross-sections, although many other highly variable shapes also occur. Also ridges range from tightly to loosely interlocking. Transverse sections appear smooth (but see Figure 5), with occasional conchoidal fractures (*).