Third declension

In the third declension, there are two subcategories: "consonant stem" and "i-stem". They are almost identical, but when it comes to the plural genitive form, nouns of the "consonant stem" get an ending "-um", while "i-stem" nouns get an "-ium" ending. How to differentiate between "consonant stem" and "i-stem"?

Nouns of "i-stem" are the following:

1. nouns ending in -is, -es, and having a genitive form with a number of syllables equal to their nominative form. E. g.: civis, civis mf, or collis, collis m;
2. nouns ending in consonant + s (-rs, -ns, -ps, -bs, -x), but only, and exclusively only, if before their genitive ending -is there are at least two consonants. E. g.: gens, gentis, f ("-nt-" is two consonants before the "-is"), or nox, noctis f ("-ct-" is two consonants before the "-is").
3. neuter nouns ending in -e, -al, -ar. E. g.: mare, maris; n, animal, -alis, n; nectar, nectaris, n.
4. almost all adjectives of the third declension

Nouns of "consonant stem" are all nouns of the third declension not matching the requirements listed above.