36.  Mos, m.  Genus, n., Honor, m.,
          custom. race. honor.

SINGULAR.
Nom.  mos             genus           honor
Gen.  moris           generis         honoris
Dat.  mori            generi          honori
Acc.  morem           genus           honorem
Voc.  mos             genus           honor
Abl.  more            genere          honore

PLURAL.
Nom.  mores           genera          honores
Gen.  morum           generum         honorum
Dat.  moribus         generibus       honoribus
Acc.  mores           genera          honores
Voc.  mores           genera          honores
Abl.  moribus         generibus       honoribus

1.  Note that the final s of the stem becomes r (between vowels) in the oblique cases.  In many words (honor, color, and the like) the r of the oblique cases has, by analogy, crept into the Nominative, displacing the earlier s, though the forms honos, colos, etc., also occur, particularly in early Latin and in poetry.

II. i-Stems.

A.  Masculine and Feminine i-Stems.

37.  These regularly end in -is in the Nominative Singular, and always have -ium in the Genitive Plural.  Originally the Accusative Singular ended in -im, the Ablative Singular in -i, and the Accusative Plural in -is; but these endings have been largely displaced by -em, -e, and -es, the endings of Consonant-Stems.

38.  Tussis, f., Ignis, m., Hostis, c.,
          cough; stem, fire; stem, enemy; stem,
          tussi-. igni-. hosti-.

SINGULAR.                          TERMINATION.
Nom.  tussis          ignis           hostis          -is
Gen.  tussis          ignis           hostis          -is
Dat.  tussi           igni            hosti           -i
Acc.  tussim          ignem           hostem          -im, -em
Voc.  tussis          ignis           hostis          -is
Abl.  tussi           igni or e       hoste           -i, -e

PLURAL.
Nom.  tusses          ignes           hostes          -es
Gen.  tussium         ignium          hostium         -ium
Dat.  tussibus        ignibus         hostibus        -ibus
Acc.  tussis or -es   ignis or -es    hostis or -es   -is, -es
Voc.  tusses          ignes           hostes          -es
Abl.  tussibus        ignibus         hostibus        -ibus

1.  To the same class belong—­

apis, bee. cratis, hurdle. +*securis, axe.
auris, ear. febris, _fever_. sementis, _sowing_.
avis, _bird_. orbis, _circle_. +sitis, thirst.
axis, axle. ovis, sheep. torris, brand.
buris, _plough-beam_. pelvis, _basin_. +turris, tower.
clavis, key. puppis, stern. trudis, pole.
collis, hill. restis, rope. vectis, lever.
and many others.