Found:

Subtractive morphology

MORPHOLOGY: a term which refers to the situation where the semantically more complex category is represented by a phonologically simpler form. EXAMPLE: the genitive plural of feminine and neuter nouns in -a/-o in Russian (kniga 'book', mesto 'place') are formed without any (overt) affixation (knig, mest).

LIT.	Scalise, S. (1984) Spencer, A. (1991)