Mantle

The downturned side of the valve surrounding the valve face. When a frustule is viewed in girdle view, the mantle is visible.

Mantle2
Image Credit: Marina Potapova
The mantle of Aulacoseira alpigena is ornamented, while the valve face has marginal arealae.
Mantle
Image Credit: Marina Potapova
The mantle of Stephanodiscus yellowstonensis is rounded at the mantle-face junction.