Which major classes are used to classify lamp shells?

Lamp shells, which belong to the phylum Brachiopoda, are classified into two major classes based on the presence or absence of articulation in their valves:12

• Class Inarticulata: These brachiopods have shells that do not articulate by teeth and sockets. Instead, their valves are held together by a complex system of muscles. Their shells are typically composed of chitinophosphatic material, and they possess an intestine with an anal opening.2
• Class Articulata: These brachiopods feature shells that articulate by means of teeth and sockets. Their shells are always composed of calcareous material, and they possess a less complicated musculature than inarticulate species. Notably, they lack an outside opening from the intestine.2