middle of the ribs with the other hand. the membrane will pull loose from both sides and come clean off.
tip: use a paper towel or catfish pliers if you need a better grip.
usually there's also a flap of meat that runs along the bone side of spare ribs. this should be cut off close to the base of the meat to allow for more even cooking. for some reason, these ribs had no flap at all so i assume the butcher removed them for some reason.
here's a picture from some other spare ribs that i have done in the past to show you how it's done. as you can see, 