This statement may have been derived either from Eusebius (Hist. Eccl.. IV. c. 5), or from the “written records” (ἐγγράφων), from which he had learned that “until the siege of the Jews which took place under Adrian (135 a.d.), there were fifteen bishops in succession there, all of whom are said to have been of Hebrew descent.”  See the list of names, and the notes on the passage in this Series.