Here is a quick taste of Paris that can be found in the Back Bay. It's not an exact replica of the Metro entrances found in the land of fromage and vin, but it certainly seems to be inspired by them. Bon!
Follow the link for images of the Parisian Metro and some interesting facts. The style is called Art Nouveau, which is apparently very difficult for me to define. It predates Art Deco - but only by little. In addition to the preceding example, it includes the architecture of Gaudi. And that's really all that I know about it.
If you'd rather an informed opinion, this link defines it as a "cluster of self-consciously radical, somewhat mannered reformist chic." I prefer the quoted definition, though I do not have the aesthetic chops to back it up. However, I do know that when I look at this picture above and read that quote, I think, "Yeah, that's about right."