4.  Impetus has the Nom., Acc., and Abl.  Sing., and the Nom. and Acc.  Plu.; viz. impetus, impetum, impetu, impetus.

5. a.  Preci, precem, prece, lacks the Nom. and Gen. Sing.

  b.  Vicis, vicem, vice, lacks the Nom. and Dat.  Sing.

6.  Opis, dapis, and frugis,—­all lack the Nom.  Sing.

7.  Many monosyllables of the Third Declension lack the Gen. Plu.:  as, cor, lux, sol, aes, os (oris), rus, sal, tus.

Indeclinable Nouns.

58.  Here belong—­

fas, n., right.               nefas, n., impiety.
instar, n., likeness.         nihil, n., nothing.
mane, n., morning.            secus, n., sex.

1.  With the exception of mane (which may serve also as Ablative, in the morning), the nouns in this list are simply Neuters confined in use to the Nominative and Accusative Singular.

Heteroclites.

59.  These are nouns whose forms are partly of one declension, and partly of another.  Thus:—­

1.  Several nouns have the entire Singular of one declension, while the Plural is of another; as,—­

  vas, vasis (vessel); Plu., vasa, vasoroum, vasis, etc.
  jugerum, jugeri (acre); Plu., jugera, jugerum, jugeribus, etc.

2.  Several nouns, while belonging in the main to one declension, have certain special forms belonging to another.  Thus:—­

  a) Many nouns of the First Declension ending in -ia take also a Nom. and
  Acc. of the Fifth; as, materies, materiem, material, as well as
  materia, materiam.

  b) Fames, hunger, regularly of the Third Declension, has the Abl. fame
  of the Fifth.

  c) Requies, requietis, rest, regularly of the Third Declension, takes
  an Acc. of the Fifth, requiem, in addition to requietem.

  d) Besides plebs, plebis, common people, of the Third Declension, we
  find plebes, plebei (also plebi, see Sec. 52, 2), of the Fifth.

Heterogeneous Nouns.

60.  Heterogeneous nouns vary in Gender.  Thus:—­

1.  Several nouns of the Second Declension have two forms,—­one Masc. in -us, and one Neuter in -um; as, clipeus, clipeum, shield; carrus, carrum, cart.

2.  Other nouns have one gender in the Singular, another in the Plural; as,—­

SINGULAR.  PLURAL.
balneum, n., bath; balneae, f., bath-house.
epulum, n., feast; epulae, f., feast.
frenum, n., bridle; freni, m.(rarely frena, n.), bridle.
jocus, m., jest; joca, n. (also joci, m.), jests.
locus, m., place; loca, n., places; loci, m., passages
or topics in an author.
rastrum, n., rake; rastri, m.; rastra, n., rakes.

a.  Heterogeneous nouns may at the same time be heteroclites, as in case
of the first two examples above.