591. In the genitive plural, a shorter form in #-um# is occasionally
found: as, #passum#, _steps_ (Plaut., Mart.); #currum#, _chariots_
(Verg.); EXERCITVM. The quantity of the #u# and the origin of this
ending are uncertain.
592. In the dative and ablative plural, the following retain #-u-bus#:
#acus#, _pin_, _needle_, #arcus#, _bow_, #partus#, _birth_, #tribus#,
_tribe_. The following have #-u-bus# or #-i-bus# (28): #artūs#, plural,
_joints_, #lacus#, _lake_, #portus#, _haven_, #specus#, _cave_, #genū#,
_knee_, #verū#, _a spit_. All other words have #-i-bus# only.