I happened to notice an ad for a used "Rowbike" on the Craigslist.
I was very curious. It's more or less a recumbent. More or less.
But - the seat slides along a horizontal rail, forward to backward. A combination lever / handlebar stem is pulled back, then released, in a rowing action to trigger the forward motion. Some sort of ratcheting mechanism apparently drives a fairly standard-looking chain/derailleur on the back wheel.
You've gotta love innovation, huh?
The Rowbike website is HERE. It includes some little video clips, so you can see the contraption in action.
My first impression: it looks like it would provide a pretty good aerobic workout, comparable with a stationary rowing machine. It would definitely exercise the upper body more than a conventional bicycle. I'd probably really enjoy riding one - for a change of pace - along a dedicated bike/pedestrian path. HOWEVER... I can't help but think that safety might be compromised by the need to "row" the thing. (But at least it's not going "backwards," like rowing a boat, huh?) Nor can I envision people going on cross-country trips on their rowbikes.