The genus Notopygos was based by Grube in 1855 ¹) on an Amphinomid worm from St. Helena, N. crinitus, especially characterized by having the anus situated dorsally at some distance from the last segment. Some time afterwards ²) he described another species, N. ornatus, from Puntarenas in Costa Rica and mentioned the presence of two dorsal cirri. Meanwhile (1857) Kinberg ³), probably unacquainted with the last named species, published a new diagnosis of the genus, in which not only the characteristical situation of the anus was not mentioned, but also added to it “cirrus dorsualis pedis unicus”. It may be presumed, that the worms collected by him in the neighbourhood of St. Helena and considered to be young specimens of N. crinitus, belonged to an other genus, because he described the caruncle as “sulco longitudinali” instead of “crista media praeditus”. Moreover lie introduced the new genus Lirione, characterized by having “cirri dorsuales utrinque bini”, in behalf of two undescribed Amphiuomidae, L. splendens from Tahiti and L. maculata from Panama. Kinberg therefore published an erroneous diagnosis of the genus Notopygos and proposed the new name Lirione for two species, undoubtedly belonging to the first named genus, that of Grube. Baird ¹) did not recognize this error, that was corrected by subsequent investigators (Ehlers, Macintosh a.o.). Nowadays about a dozen of species of Notopygos are described, but, as is rightly stated by Potts ²), they are unfortunately rather ill-defined and so it is sometimes a very difficult, if not an impossible task to recognize the species. Potts found that one or more of the anterior segments always possess denticulated setae, even when in an other region of the body they are not present. I for one think however, that the presence or absence of serrations in the setae is not the only character that must be taken into account, as, like in Chloeia, the branchiae do not seem to commence always on the same segment and also the number of the folds of the caruncle and the situation of the anal pore differs in different species, though unfortunately in several cases this is not mentioned by the authors. In the following table the hitherto described species of Notopygos are enumerated, especially in order to demonstrate the numerous gaps in our knowledge of this genus.