If you have a bar clamp wide enough, squeeze the shell perpendicular from the split to open it wide enough to fill with glue. Flexible glue (e.g. tacky craft glue) is better than hard glue (e.g. Titebond). Most glue can be thinned with water to be sure it flows into the crack. After glue is applied, release the clamp & squeeze the area tightly. It helps to have something pliable between the shell and clamp like cardboard or layers of veneer glued together to distribute the pressure evenly.