[*] 45. The Stem of nouns of the Second Declension ends in ŏ-: as, viro- (stem of vir, man), servo- (stem of servus or servos, slave), bello- (stem of bellum, war). [*] a. The Nominative is formed from the stem by adding s in masculines and feminines, and m in neuters, the vowel ŏ being weakened to ŭ (see §§ 6. a, 46. N.1). [*] b. In most nouns whose stem ends in rŏ- the s is not added in the Nominative, but o is lost, and e intrudes before r,1 if not already present: as, ager , stem agrŏ- 2; cf. puer , stem puero-. Exceptions: erus , hesperus, iūniperus , mōrus , numerus , taurus , umerus , uterus, vīrus, and many Greek nouns. [*] c. The stem-vowel ŏ has a variant form ĕ,3 which is preserved in the Latin vocative singular of nouns in -us: as, servĕ, vocative of servus, slave.
[*] Note.--In composition this ĕ appears as ĭ. Thus,—belli-ger, warlike (from bell- o/e-, stem of bellum, war).