A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... 190s 200s 210s 220s 230s - 240s - 250s 260s 270s 280s 290s Years: 237 238 239 240 241 - 242 - 243 244 245 246 247 Events Patriarch Titus[?] succeeds ...