What is not consisting of an equal number of syllables; as, an imparisyllabic noun, one which has not the same number of syllables in all the cases; as, lapis, lapidis; mens, mentis called?

imparisyllabic — Not consisting of an equal number of syllables; as, an imparisyllabic noun, one which has not the same number of syllables in all the cases; as, lapis, lapidis; mens, mentis. (Webster's 1913.)

Sources