Frustules in which one valve differs from the other morphologically. Frustules may be heterovalvar in their valve ornamentation or their raphe system. The term is often applied to the monoraphid diatoms, in which one valve has a normal raphe and the other is rapheless.

Platessa Conspicua Example
Image Credit: Marina Potapova
The species Platessa inconspicua is an example of a heterovalvar diatom. One valve has a raphe system and the other valve lacks it. This species is also heterovalvar because of such characters as the morphological differences of the central areas of the two valves.