§90. Gender in the Fourth Declension
Most nouns of the Fourth Declension in -us are Masculine.
Exceptions: The following are Feminine: acus, anus, colus, domus, īdūs (plural), manus, nurus, porticus, quīnquātrūs (plural), socrus, tribus, with a few names of plants and trees. Also, rarely, penus, specus.
J. H. Allen and J. B. Greenough, New Latin Grammar (Boston: Ginn, 1903). Text provided under a CC BY-SA license by Perseus Digital Library, http://www.perseus.tufts.edu, with funding from the National Endowment for the Humanities.