Frustules in which one valve differs in morphology from the other. Many of the diatoms are heterovalvar based on valve ornamentation or the presence or absence of a raphe system. The term is often used to refer to the monoraphid diatoms, in which one valve possesses a normal raphe valve and the other possesses a rapheless valve (that has been secondarily filled with silica).
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. In addition to being heterovalvar because of the raphe system, this species is also heterovalvar based on differences in morphology between the two valves of a frustule.