OK I got the last one - I'll take you through what I did as it could help you in the future with your 50!.
Firstly - look at the bust, the style of this one is 'laureate' (the wreath on his head), 'draped' (clothing on his shoulders) and 'cuirassed' (Amour - cant really see it but I think its there on the centre of his chest'.
The bust can help date the coin to a period though not exclusively - For example the above is very common with Constantine and the emperors around him in date. Likewise if the the Bust had a 'radiate' (Spikey crown) then it is prob from 215 - end of the 3rd century. It helps cut down the names we are looking for.
This is not an exact science as there will be many coins that 'break the rule' but is a good start.
So looking closer at the text on yours one we can see 'CONSTAN' quite clearly - it helps as we already know it to be of the Constantine era as it means we look for certain letters in the text - it seems to help them stand out.
Unfortunately there are a lot of Constan...'s (Constantine I, II and II, Constantius I and II and Constans!).
The best thing to do here is start with Constantine I - he was on the throne for 30 (307-337) years and is far more common than the rest due to the number of coins produced under him.
Pop over to Wildwinds.com and search his coins with thumbnails (you need to click at the top where it asks about this):
http://wildwinds.com/coins/ric/constantine/t.htmlStraight away we see the resemblance of the bust - use the thumbnails on the right to look for something that resembles the reverse of your coin. Luckily we dont have to go far - the 9th one down looks very close:
http://wildwinds.com/coins/ric/cons..._vII_058.jpgCompare the text on the two and we can see it fits on the Legend where there is a gap in it CONSTANTI---NVS MAX AVG. It also matches on the Reverse with the gaps GLOR---IA EXERC---ITVS.
Cool - we got our match.
If it doesn't fit you just have to keep going till you find one that does - sometimes you can guess the missing letter by fitting them to Legends that are known, also the number of missing letters can help you remove emperors from the ones you are searching - eg if the name would be too long or short to fit.
Sorry if I've gone over stuff you know but it was easier to write from start to finish and I'm sure some of it was helpful.
CONSTANTI-NVS MAX AVG, rosette-diadem, draped, cuirassed bust right GLOR-IA EXERC-ITVS, two soldiers holding spears and shields with two standards between them.
I'll post something about the mint mark in a minute.