Complex sentence: The Memoirs of Prince Dolgoroukoff assert that the Swiatopelk-Mirskis were princes descending from Rurik who submitted to Gediminas & became magnates.The Genealogisches Handbuch der Fürstlichen Häuser states that the Swiatopolk-Mirsky family descends from a younger branch of the Princes of Turov.
Split into simpler sentences: The Memoirs of Prince Dolgoroukoff assert that the Swiatopelk-Mirskis were princes descending from Rurik who submitted to Gediminas & became magnates. The "Genealogisches Handbuch der fürstlichen Häuser" states that the Swiatopolk-Mirsky family descends from a younger branch of the Princes of Turov.