Institutio Oratoria

The same scholars, however, when asked why aper became apri in the genitive, but pater patris, asserted that aper was an absolute, pater a relative noun. Further since both words derive from the Greek, they took refuge in the fact

But how will they evade the difficulty that feminine nouns whose nominative singular ends in -us never make the genitive end in -ris, and yet the genitive of Venus is Veneris: again nouns ending in -es have various genitive terminations, but never end in -ris, but yet we have no choice but to make the genitive of Ceres Cereris?