On Aeneid II.1(Conticuēre omnes intentique ora tenebant), Servius notes:
conticvere omnes quia supra dixit "fit strepitus tectis". 'conticuere' autem pro conticuerunt: quod metri causa fit vel ratione clausularum.
According to Servius, the conticuēre form serves for metical purposes and/or as a (rhetorical?) tool he refers to as ratio clausularum which I could not really find what it means.
A clausula is "a rhythmic figure used to add finality to the end of a sentence or phrase", to quote Wikipedia. ratio clausularum looks simply to mean something like 'for reasons to do with the clausulae' or 'for the sake of the clausulae'.
Perhaps ratio should be taken as 'order, conformity' rather than 'cause', in which case the phrase could be interpreted as 'to form a pretty sequence of clausulae'. It amounts to the same thing in the end.
