Pelophylax is a genus of true frogs widespread in Eurasia, with a few species ranging into northern Africa. They are also known as Water Frogs, as they spend much of the summer living in aquatic habitat.
Most authors throughout the 19th and 20th century disagreed with Fitzinger's assessment. The Green Frogs were included again with the Brown Frogs, in line with the tendency to place any frog similar in habitus to the Common Frog in Rana. That genus, in the loose circumscription, eventually became a sort of "wastebin taxon".
Around the year 2000, with molecular phylogenetic studies becoming commonplace, it was discovered that Fitzinger's assessment was correct after all – not only is Pelophylax an independent genus, but it does in fact belong to a lineage of Raninae not particularly close to Rana. But it also turned out that these Eurasian Green Frogs might not form a monophyletic lineage.
Source: en.wikipedia.org/wiki/Pelophylax - 20.05.2010
The Water Frog has - contrary to the one central vocal sac with the Tree Frogs - two vocal sacs which are splendidly shown on the picture.