Abstract
Small naviculoid diatoms remain largely taxonomically unexplored in the United States, due to the inability for clear
differentiation of unique characteristics under light microscope magnification during routine identification. In a recent study of benthic stream diatoms from California and as a part of taxonomy data quality assurance and quality control process it became evident that taxonomic reevaluation of a common Fallacia species is needed. A new small-celled Fallacia species is described here and compared to similar taxa. Nomenclatural and ecological discussions, based on historical and current literature, are presented for each taxon. Type materials were consulted for taxonomic evaluation and comparison. The new taxon is described with light microscopy and scanning electron microscopy. Fallacia californica could be distinguished from other small Fallacia species with coarse areolae by the unique combination of following features: 1) distinct lyre-shaped sternum, 2) asymmetrical central area, and 3) variable central striae with one to four inner areolae absent, and unilateral gap between the central striae on the mantle. The new freshwater species was distributed in warm alkaline waters with medium conductivity and elevated nutrients content.The majority of populations have been recorded in coastal streams, where they could be abundant, but the species could be scattered in other inland running waters as well.