One can often derive adjectives from city names, the most famous example probably being Romanus from Roma. Such derivatives are typically formed with -anus or -ensis. My impression is that -anus is typically restricted to names ending in -a (like Roma), but -ensis can be used for any kind of name. For a first declension example, the adjective derived from Aboa (Turku) is Aboensis, not Aboanus.
Suppose I have a city name ending in -a. Is there any rule or tendency for choosing between -anus and -ensis? How do I know which one to pick? Does it depend more on the time and location where the derivative is formed or on the type of the city?
This is related to the recent question about Eura. I learned that the city is called Eura in Latin, but now I would like to know whether to derive Euranus or Eurensis. However, instead of this special case I want to focus on general rules (or lack thereof) for such derivations. If someone knows the answer for Eura, I can make it into a separate question so that particular translation stands out (or you can post it as an answer to the previous Eura question).
