Maa Siddhidatri is definitely the ninth avatar of maa Durga and it is worshipped around the 9th working day of navratri. Ma Siddhidatri is represented with 4 arms sitting on a lovely lotus flower. While in the Srimad Bhagavatam, a summary of fourteen various names are offered for Durga. Similar https://www.youtube.com/watch?v=S1HpMpEww4Q