Morphology and taxonomy of Amphicocconeis gen. nov (Achnanthales, Bacillariophyceae, Bacillariophyta) with considerations on its relationship to other monoraphid diatom genera

SEM and TEM observations on the cleaned material used by Hustedt for the preparation of the type of the marine diatom Cocconeis disculoides Hustedt and on living specimens of the species scraped from Posidonia oceanica (L.) Delile leaves collected in different coastal areas of the Mediterranean Sea revealed structures that differentiate this species from the genus Cocconeis Ehrenberg. The external apical raphe endings have marked terminal fissures; valvocopulae extend over a large part, or all, of the valve surface and bear rounded perforations arranged differently in the two bands; areolae are occluded by simple hymenes with irregularly distributed, rounded perforations. Recently, C. disculoides Hustedt was transferred to Psammococconeis Garcia, a new genus found in sediment samples from Port Belo Peninsula (Brazil), as P. disculoides (Hustedt) Garcia. However, the genus Psammococconeis lacks the raphe terminal fissures, shows two types of areolae in the sternum valve mantle and an extended valvocopula with perforations only in the raphid valve. Furthermore, in P. disculoides, the areolae are occluded by fimbriate hymenes in both valves. These observations lead us to propose the new genus Amphicocconeis, which at present includes only A. disculoides (Hustedt) comb. nov. The elements of Hustedt's type material examined by Garcia should be the lectotype of a new species of Psammococconeis.