WHY IS IT NOT A "SHELL"

[diocrisseashells1000]

thanks dave for your answer,but im still confused why not include it in the family of mollusk? there are still living brachiopods more than the the class monoplachopora. (correct me if im wrong). you can only see records of monoplachopora and never seen a live or collected shell of it nor anybody show a picture of one shell,as they say its "extinct".does the anatomy of a mollusk differs of  brachiopods will base the fact it cannot be included in the family? what about their characteristics?
i think scallops and lampshell both do the same kind of feeding and dwelling.

perhaps our scientist is so very intelligent ???, we cannot understand or question them on how they classify or group the living things in the ocean. ???  :-X
if they are out there?...... please,......can you explain it..... > >
thanks again dave and happy shelling