92. The following peculiarities in case forms of the 4th Declension require notice.
a. A genitive singular in -ī (as of the 2nd declension) sometimes occurs in nouns in -tus.
senātus, gen. senātī (regularly senātūs)
b. In the genitive plural -uum is sometimes pronounced as one syllable, and may then be written -um.
currum (Aen. 6.653) for curruum
c. The dative and ablative plural in -ŭbus are retained in partus and tribes, regularly in artus and lacus, and occasionally in other words; portus and specus have both -ubus and -ibus.
d. Most names of plants and trees, and colus (distaff) also have 2nd declension forms.
fīcus fig, gen. fīcūs or fīcī
e. An old genitive singular in -uis or -uos and an old Genitive plural in -uom occur rarely.
f. The ablative singular ended anciently in -ūd (cf. § 43, Note 1): magistrātūd.
Note 1— The Locative is domī (rarely domuī) at home.
Note 2— The Genitive domī occurs in Plautus; domōrum is late or poetic.