Naviculoid refers to diatoms in the genus Navicula or are similar in shape to Navicula. Naviculoid means "resembles Navicula", so it is not a formal term. It could refer to any diatom that is symmetric and biraphid.

A naviculoid valve margin has the shape of a boat. The genus Navicula was described in 1822 by Bory for having this shape.

Boat Shape
Image Credit: Mark Edlund
Living cell of Navicula showing the cell cytoplasm, including the marginal chloroplasts. The genus was named for having a naviculoid shape.