A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... Contents 242 Centuries: 2nd century - 3rd century - 4th century Decades: 190s 200s 210s 220s 230s - 240s - 250s 260s 270s 280s 290s Years: 237 238 239 240 ...