Allen and Greenough's New Latin Grammar, section 181

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Peculiarities of Conjugation.

181.

In tenses formed upon the Perfect Stem, v between two vowels is often lost and contraction takes place.

a. Perfects in -áví, -éví, -óví, often contract the two vowels into á, é, ó, respectively: as, amásse for amávisse; amárim for amáverim; amássem for amávissem; cónsuérat for cónsuéverat; fléstis for flévistis; nósse for nóvisse. So in perfects in -ví, where the v is a part of the present stem: as, commórat for commóverat.

NOTE: The first person of the perfect indicative (as, amáví) is never contracted, the third very rarely.

b. Perfects in -íví regularly omit v, but rarely contract the vowels except before st and ss, and very rarely in the third person perfect: -

audieram for audíveram; audísse for audívisse; audístí for audívistí; abiit for abívit; abiérunt for abívérunt.

NOTE 1: The forms síris, sírit, sírítis, sírint, for síveris etc. (from síveró or síverim), are archaic.

NOTE 2: In many forms from the perfect stem is, iss, sis, are lost in like manner, when s would be repeated if they were retained: as, díxtí for díxistí (x cs); tráxe for tráxisse; évástí for évásistí; víxet for víxisset; érépsémus for érépsissémus; décésse for décessisse. These forms belong to archaic and colloquial usage.