49.a. The locative form of this declension ends for the singular in -ī.

humī  on the ground 
Corinthī  at Corinth

For the plural, in -īs.

Philippīs  at Philippi (cf. § 80, footnote)

b. The genitive of nouns in -ius or -ium ended, until the Augustan Age, in a single -ī;

fīlī  of a son 
Pompêī  of Pompey (Pompêius)

but the accent of the Nominative is retained.

ingĕ'nī  of genius¹

c. Proper names in -ius have -ī in the Vocative, retaining the accent of the Nominative.

Vergĭ'lī

So also, fīlius (son); genius (divine guardian).

Audī, mī fīlī.  Hear, my son.

Adjectives in -ĭus form the Vocative in -ie, and some of these are occasionally used as nouns.

Lacedaemonie  O Spartan

Note— Greek names in -īus have the vocative -īe.

Lyrcīus; voc. Lyrcīe

d. The genitive plural often has -um or (after v) -om (cf. § 6.a) instead of -ōrum, especially in the poets:

deum, superum, dīvom  of the gods 
virum  of men

also in compounds of vir, and in many words of money, measure, and weight.

Sēvirum  of the Seviri 
nummum  of coins 
iūgerum  of acres

e. The original ending of the Ablative singular (-ōd) is sometimes found in early Latin.

Gnaivōd (later, Gnaeō), Cneius

f. Proper names in -âius, êius, -ôius (as, Aurunculêius, Bôī), are declined like Pompêius.

g. Deus [(m.), god] is thus declined.

Note— The vocative singular of deus does not occur in classic Latin, but is said to have been dee; deus (like the nominative) occurs in the Vulgate. For the genitive plural, dīvum or dīvom (from dīvus,  divine) is often used.

Footnotes