As a vocative form, deus is a clear violation of established norms. The grammarians occasionally encountered apparent examples of such vocatives in the texts they taught, and they explained them either by invoking the figure of antiptosis, in which the “correct” case is replaced by another,²⁸ or by appeal to the concept of euphonia, which allowed that some forms dictated by the rules could be avoided because they struck the ear as unpleasant, or, in the case of poetic texts, by arguing that the regular vocative could not be used because of metrical restraints. 28. For a discussion of the role of figurae in the grammarians’ explication of classical texts, and of antiptosis in particular, see R. A. Kaster, Guardians of Language: The Grammarians and Society in Late Antiquity (Berkeley, 1988), 174–76.
ibidem, page 144:
Priscian accordingly does not regardfluviusat Aeneid 8.77 orpopulusat Lucan 2.116 as actual vocatives, but instead treats them as examples of antiptosis (Keil II 305.16-21).³⁰ 30.Fluviusat Aen. 8.77 is also treated as antiptosis by other grammarians who saw in Virgil’s avoidance of the vocative a recognition of what had for them become the hotly debated issue of the consonantal i, as shown, for example, by the comments that appear in theExplanationes in Artem Donatiattributed to Sergius:“vocativus secundae declinationis aliquando in i exit, aliquando in e. denique in hoc casu ambiguitatem vidit Vergilius et antiptosim fecit, pro vocativo nominativum posuit . . . et hic usus veterum fuit”(Keil IV 498.17–22). A similar interpretation offluviusis presented by Servius himself in hisCommentarius in artem Donati(Keil IV 409.15–18), and by the grammarian Phocas (Keil V 429.15–18).
ibidem, page 145:
Priscian’s argument is essentially ahistorical. He presentsdeushere as a contemporary form, one that is familiar to students and that can be used to illustrate the violation of grammar that occurs at Aeneid 8.77, because, in his view, the veteres employed antiptosis for reasons of euphony in just the same way contemporary speakers did(“ut deus pro dee et fluvius pro fluvie, ut Virgilius in VIII Aeneidos”).