The 14th-century music theorist Jacobus devoted a complete chapter of his Speculum musicae (SM vii.37) to a critique of what he termed ‘solitary’ semibreves (semibreves solitarias). He listed several arguments against the moderns’ use of solitary semibreves. The present essay considers what this motet repertory that used solitary semibreves might be. The emergence of the Ars Nova is often identified with the newer motets copied in the Bibliothèque Nationale de France fr. 146 manuscript of the Roman de Fauvel, but solitary semibreves are not found in the motets of Fauvel. In this article, I examine some ambiguities of the semibreve notation in BnF fr. 146. I then consider the evidence of another group of motets classified as Ars Nova motets by music historians since they are explicitly notated in Ars Nova notation in manuscript sources copied later than BnF fr. 146. These motets contain clues indicating that they were originally copied in a notation similar to that used in the Roman de Fauvel. That is, these motets too (originally) had no solitary semibreves. To find the solitary semibreves of Jacobus’s complaint then, and per consequens, the music of Jacobus’s ‘ars nova’, we must identify compositions that appear to have been originally conceived in Ars Nova notation. This article closes with a brief consideration of some of these motets.