§45. SECOND DECLENSION (o-STEMS)
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.
J. H. Allen and J. B. Greenough, New Latin Grammar (Boston: Ginn, 1903). Text provided under a CC BY-SA license by Perseus Digital Library, http://www.perseus.tufts.edu, with funding from the National Endowment for the Humanities.