In this declension the Base of the Noun often appears in an altered form in the Nom. Sing. Thus the base of lĕō lion is lĕōn-, of hŏmo man, hŏmĭn-, of gĕnŭs kind, gĕnĕr-, of corpŭs body, corpŏr-. In order to decline a Noun of this declension it is not sufificient to know that it is of the Third Declension : we must also learn what its Gen. Sing. is.
Gender.—Various. For detailed rules see § 263.
Genitive Plural.—(1) Parisyllabic or Non-increasing Nouns, that is, Nouns which have the same number of syllables in Gen. Sing. as in Nom., have -ĭum in Gen. Plur.
(2) Imparisyllabic or Increasing, Nouns which have more syllables in Gen. Sing. than in Nom., have -um in Gen. Plur.
(3) But Nouns whose Nom. is Monosyllabic take -ĭum if the base ends with two consonants.
Note.—Pătĕr father, mātĕr mother, frātĕr brother, are only apparently Parisyllabic, as they have lost an e in the Gen. Sing., and so make Gen. Plur. pătr-um, mātr-um, frātr-um.
Masculine and Feminine Nouns.—Exx. (1) host-ĭs Gen. host-ĭs (m.) enemy; (2) cōnsŭl, Gen. cōnsŭl-ĭs (m.) consul; (3) ar-s, base art-, Gen. art-ĭs (f.) art.
| Sing. | Plur. | |
| N.V. | host-ĭs | host-ēs |
| A. | host-em | host-ēs |
| G. | host-ĭs | host-ĭum |
| D. | host-ī | host-ĭbŭs |
| Abl. | host-ĕ | host-ĭbŭs |
| Sing. | Plur. | |
| N.V. | cōnsŭl | cōnsŭl-ēs |
| A. | cōnsŭl-em | cōnsŭl-ēs |
| G. | cōnsŭl-ĭs | cōnsŭl-um |
| D. | cōnsŭl-ī | cōnsŭl-ĭbŭs |
| Abl. | cōnsŭl-ĕ | cōnsŭl-ĭbŭs |
