Allen and Greenough's New Latin Grammar, section 78

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78.

The following have -um (not -ium) in the genitive plural.

1. Always, - canis, iuvenis,[1][Canis and iuvenis are really n-stems.] ambágés, mare (once only, otherwise wanting), volucris; regularly, sédés, vátés.

2. Sometimes, - apis, caedés, cládés, ménsis, strués, subolés.

3. Very rarely, - patrials in -ás, -átis; -ís, -ítis; as, Arpínás, Arpínátum; Samnís, Samnítum.