In the Nominative and Vocative Singular of ager, the stem is further
modified by the development of e before r.

2. The following nouns in -er are declined like puer: adulter, _adulterer_;
gener, _son-in-law_; Liber, _Bacchus_; socer, _father-in-law_; vesper,
_evening_; and compounds in -fer and -ger, as signifer, armiger.

Nouns in _-vus_, _-vum_, _-quus_.

24. Nouns ending in the Nominative Singular in -vus, -vum, -quus, exhibited
two types of inflection in the classical Latin,--an earlier and a
later,--as follows:--

_Earlier Inflection (including Caesar and Cicero)._
Servos, m., Aevom, n., Equos, m.,
_slave_. _age_. _horse_.
SINGULAR.
_Nom._ servos aevom equos
_Gen._ servi aevi equi
_Dat._ servo aevo equo
_Acc._ servom aevom equom
_Voc._ serve aevom eque
_Abl._ servo aevo equo

_Later inflection (after Cicero)._
SINGULAR.
_Nom._ servus aevum equus
_Gen._ servi aevi equi
_Dat._ servo aevo equo
_Act._ servum aevum equum
_Voc._ serve aevum eque
_Abl._ servo aevo equo

1. The Plural of these nouns is regular, and always uniform.

Peculiarities of Inflection in the Second Declension.

25. 1. Proper names in -ius regularly form the Genitive Singular in -i
(instead of -ii), and the Vocative Singular in -i (for -ie); as Vergili,
_of Virgil_, or _O Virgil_ (instead of Vergilii, Vergilie). In such words
the accent stands upon the penult, even though that be short. Nouns in
-ajus, -ejus form the Gen. in -ai, -ei, as Pompejus, Pompei.

2. Nouns in -ius and -ium, until after the beginning of the reign of
Augustus (31 B.C.), regularly formed the Genitive Singular in -i (instead
of -ii); as,--

_Nom._ ingenium filius
_Gen._ ingeni fili

These Genitives accent the penult, even when it is short.

3. Filius forms the Vocative Singular in -i (for -ie); _viz_. fili, _O
son!_

4. Deus, _god_, lacks the Vocative Singular. The Plural is inflected as
follows:--

_Nom._ di (dei)
_Gen._ deorum (deum)
_Dat._ dis (deis)
_Acc._ deos
_Voc._ di (dei)
_Abl._