Hood
The hood is an internal silica extension that extends over the rimmed depression, found in species within the genus Planothidium.
Alternative Term
cavum
Several references (including Kingston 2003) use the Latin term "cavum" to describe this structure.
Citations
Kingston, J.C. (2003). Araphid and Monoraphid Diatoms. In: Freshwater Algae of North America. Ecology and Classification (J.D. Wehr and R.G. Sheath, eds). Elsevier Science pp. 595-636.
Image Credit: Marina Potapova
This scanning electron micrograph shows the internal view of the rapheless valve of a species in Planothidium. The hood is a rounded silica structure.
Image Credit: Marina Potapova
The hood can also be viewed in the light microscope. Here the rimmed depression and the edge of the hood is visible.