The intricately sculptured unit at each end of the diatom
The broad surface displaying most of the ornamentation is called the face.
The downturned surface that meets the
is called the mantle.
The morphology of the valve provides many of the features used to classify diatoms.
Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.