Actually managed to stitch some clamshells tonight.
This will be done in Oriental Fabrics, hopefully a bedsized quilt. The Scot says ... well that's going to keep you quiet.
Thanks to those who replied re the seam allowance.
Another question ..... does it really matter whether they are cut on the straight or slightly on the bias of the fabric. The reason being I have some odd shaped pieces which could fit one shell on the bias but not on the straight.