562. In the plural, gender nominatives have #-ēs#, rarely #-īs# or
#-eīs#, and gender accusatives have #-īs# or #-ēs# indifferently,
sometimes #-eis#; after about 50 A.D., #-ēs# was the prevalent ending
for both cases. Neuters add #-a# to the stem, making #-ia#; for #-iā́#
in old Latin, cf. 2505.
563. In the genitive plural, present participles, some substantive stems
in #-nt(i)-#, and some adjectives ‘of two endings’ (631) have
occasionally #-um#: as,
#amantum#, _lovers_; #rudentum#, _rigging_; #agrestum#, _country folk_;
#caelestum#, _heaven’s tenantry_. #apis#, _bee_, has commonly #-um#;
#caedēs#, _slaughter_, and #fraus#, _deceit_, have rarely #-um#. For
#-um# in some adjectives ‘of one ending,’ see 636; for #-bū́s# in the
dative and ablative in old Latin, see 2505.