Pennate diatoms having a raphe on only one valve. A monoraphid diatom is a type of heterovalvar diatom. Example genera include Cocconeis, Achnanthidium, and Eucocconeis.

During the ontogeny of monoraphid diatoms, a raphe begins to form on each valve. One valve completes its raphe and becomes the raphe valve. The other valve fills its incipient raphe with silica, leaving a thickened axial rib, and becomes the rapheless valve.

The genus Nupela includes some monoraphid species and some biraphid species. In this web flora, Nupela is included in the biraphid group.

P Curtissimum Rapheless 1
Image Credit: Marina Potapova
These scanning electron micrographs show the rapheless valve and raphe valve of Psammothidium curtissimum. This heterovalvar species is an example of a monoraphid diatom.