Peculiarities of Nouns of the Fourth Declension.
[49]. 1. Nouns in -us, particularly in early Latin, often form the Genitive Singular in -ī, following the analogy of nouns in -us of the Second Declension; as, senātī, ōrnātī. This is usually the case in Plautus and Terence.
2. Nouns in -us sometimes have -ū in the Dative Singular, instead of -uī; as, frūctū (for frūctuī).
3. The ending -ubus, instead of -ibus, occurs in the Dative and Ablative Plural of artūs (Plural), limbs; tribus, tribe; and in dis-syllables in -cus; as, artubus, tribubus, arcubus, lacubus. But with the exception of tribus, all these words admit the forms in -ibus as well as those in -ubus.
4. Domus, house, is declined according to the Fourth Declension, but has also the following forms of the Second:—
domī (locative), at home;
domō, from home;
domum, homewards, to one's home;
domōs, homewards, to their (etc.) homes
5. The only Neuters of this declension in common use are: cornū, horn; genū, knee; and verū, spit.