A silica extension with the shape of a hood on the inside of a valve. It occurs only in some species of Planothidium where it partly covers the inside of the asymmetrical central area of the rapheless valve. See also rimmed depression.

Alternative Terms

Several references, including Kingston 2003, use the Latin term "cavum" to describe this structure.
Planothidium Hood
Image Credit: Marina Potapova
A scanning electron micrograph of the inside of the rapheless valve of a species of Planothidium. The rounded, siliceous structure at the arrow is the hood.
Mo000465 2B Ed
Image Credit: Marina Potapova
The rimmed depression and the edge of the hood are visible in this light micrograph. The arrow rests on the open edge of the hood.