40-50F is a huge range depending on other conditions. Sunny, calm 50F? Arm and knee warmers. Might even end up pulling down the arm warmers if I'm riding hard. Rain and wind 40F? Could be tights, merino baselayer, skullcap, ski gloves, booties, the whole nine yards.

But generally: arm warmers; knee warmers; and a wind vest. My favorite vest has wind proof fabric on the front and mesh on the back, a high collar, a two-way zipper, and proper pockets.