Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

[*] b. The genitive of nouns in -ius or -ium ended, until the Augustan Age, in a single -ī: as, fīlī, of a son; Pompêī, of Pompey ( Pompêius ); but the accent of the nominative is retained: as, ingĕ'nī, of genius. ¹

[*] c. Proper names in -ius have -ī in the vocative, retaining the accent of the nominative: as, Vergĭ'lī. So also, fīlius, son; genius, divine guardian: as, audī , mī fīlī, hear, my son.

Adjectives in -ĭus form the vocative in -ie, and some of these are occasionally used as nouns: as, Lacedaemonie, O Spartan.

[*] Note.--Greek names in -īus have the vocative -īe: as, Lyrcīus , vocative Lyrcīe .

[*] d. The genitive plural often has -um or (after v ) -om (cf. § 6. a) instead of -ōrum, especially in the poets: as, deum , superum , dīvom, of the gods; virum, of men. Also in compounds of vir , and in many words of money, measure, and weight: as, 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: as, 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.