Devold Merino Wool Tights, Sport
This is what we wear when it gets below about 48F. They're snuggish 100 percent wool longies, and although they look like tights, they aren't as snug as Pearl Izumi tights, for instance. They don't snap on, and you don't have to turn them inside out to take them off. They are perfect for all cool to cold weather non-racing riding.
You can wear them over your panties (the all wool ones are the best) with nothing on top, or wear MUSA shorts, knickers, or pants on over them; or nylon gym shorts, whatever. They're about a 190g/m interlock. They seem to last a long time, and do fantastic quadruple duty as pajama bottoms (every bike campout, this is what I sleep in), and running tights and hiking tights.
These are an integral component of any cold weather rig.