A pointed, silica extension of the valve, shorter than a seta. Spines and may be solid or hollow, very long or tiny, single or many, and they may arise from different points on the valve in different taxa.

Yellowstone Vcd 031 Copy
Image Credit: Marina Potapova and Sarah Spaulding
Valves of Stephanodicus have many marginal spines. The spines are solid, rather than hollow.
Staurosirella Pinnata Gsn92019  Cook 2001 Scale 5 Microns 7 1039
Image Credit: Eduardo Morales
Staurosirella has spines that are positioned in the costae, or thickened silica ridges, of the valve. These spines are hollow and forked at the tip.
Image Credit: Mark Edlund
Setae is a more specialized term, but could also be applied to the long spines of Chaetoceros elmorei. The setae are considered to aid in maintaining bouyancy of this planktonic species.