The genus name Nanogona Cook was proposed in 1895, with Craspedosoma polydesmoides Leach 1814 as its type species. Polymicrodon Verhoeff was proposed in 1897, with Atractosoma latzeli Verhoeff 1891 as its type species.
The argument for synonymy rests on Brolemann’s 1935 statement that C. polydesmoides is a senior subjective synonym of A. latzeli. According to Jeekel’s 1970 Nomenclator, that statement is incorrect. Therefore the species polydesmoides Leach 1814 should be in the genus Nanogona, and Polymicrodon Verhoeff 1897 is not a synonym of Nanogona Cook 1895, but—again according to Jeekel—Polymicrodon is a junior synonym of Grypogona Cook 1895, since both were based on the same type species. That’s how it stands at present.
The species polydesmoides and latzeli should, however, be checked to finally resolve the question