No pix snail ID

[Scavenger]

I've been seeing what I've been calling "biker snails" in my tank. I haven't been able to get a good picture of them yet, but they have shells that remind me of the dish helmets worn by a lot of harley riding bikers, leaving alot of body exposed. (hence the name biker snails.) They are about 1/4-1/2 inch long and range from grey to black and seem to graze algae so I haven't tried to remove them. Anyone got any idea? I think they are harmless, but just wanted to put a name to a face (so to speak )