The headword here is in the nominative singular, but the only non-lexicographical attestation of the term is in the accusative singular at Aristophanes, Birds 1403 (web address 1), from which all surviving commentary on the word probably starts (cf. scholia ad loc., Hesychius kappa4474).
[1] 'Lagos the Interpreter' should be lambda 139: Lasos of Hermione (*la/sos o( *(ermiwneu/s), a sixth-century BCE poet credited at [Plutarch], De musica 1141C with inventing the dithyramb (see next note).
[2] 'Circular dances' or 'circular choruses' was another way to refer to the type of song-and-dance performance known as the dithyramb; see scholia ad loc.