The genus Planothidium Round & Bukhtiyarova ex Round (1996) was originally described in 1996 to accommodate monoraphid diatom species with multiseriate striae and long terminal raphe fissures (Round & Bukhtiyarova 1996, Morales 2006, Jahn et al. 2017, Wetzel et al. 2019). Morales (2006) proposed to subdivide the genus into four groups based on the shape and structure of the central area of the sternum valve (rapheless valve). Two of the groups present a peculiar asymmetrical structure in the central area whereas the two other groups lack this structure. One group possesses the so-called sinus, a shallow, rounded hyaline depression in the central area, and contains species such as P. lanceolatum (Brébisson ex Kützing 1846: 247) Lange-Bertalot (1999: 287). The second group also shows this depression in the central area, but the area is additionally covered by a thick, often broad silica hood, called a cavum. Typical species in this group include P. frequentissimum (Lange-Bertalot 1993: 4) Lange-Bertalot (1999: 282) and P. victorii P.M.Novis et al. (2012: 22).