This group includes the brachiopods, the "lamp shells," named that because one form is similar to the form of an oil lamp like Aladdin's lamp. Brachiopods are shelled organisms, with the shell made of calcite. They might superficially look like clams, but they are a totally separate group (see the branching pattern of evolutionary relationship between brachiopods and Mollusca, the group that includes clams). Plus, whereas the shells of clams are left and right, the shells of brachipods are top and bottom, or, more technically, dorsal and ventral. There is also a kind of "stalk" that comes out of a hole in the shell near the hinge. This "stalk," called the pedicle, is made of fleshy, ligamentous material that serves to hold the brachiopod to the bottom.
There are two major groups of brachiopods, the inarticulate and articulate brachiopods. The names don't refer to the intelligence of speech in brachiopods, but to the way the hinge between the dorsal and ventral parts of the shell articulate (articulate = move, or connect, at a joint, or junction). The articulate brachiopods were, by far, more diverse, and were dominant on the sea bottom during the Paleozoic Era. There are still a few groups of brachiopods around today, and they were around in the Mesozoic Era, as well, but for the most part, brachiopods had their heyday during the Paleozoic Era.
Lingula is a long-lived brachipod, still around today. The following specimen is from the Ordovician of the Philippines:
Image Source: University of Oslo, Paleontological Museum
Modern Lingula illustrate the habits of the group, attached to the bottom by the pedicle, positioned off the sea bottom where they can open up their shell a bit to allow water and food particles to be brought in to the mouth, as you can see in this aquarium photograph .
Some Paleozoic rocks are jamb-packed with brachiopods. This specimen has many pieces of broken shells, and some complete, but much of the material is from brachipods:
Whole brachiopods can be seen in this slab of limestone from the Ordovician of Indiana:
Image Source: Wikipedia
The following photographs show the varied form of articulate brachiopods from the Paleozoic Era -- this was a diverse group!
The one on the bottom right is positioned so that you have a clear view of the long hinge line, characteristic of many brachiopod groups.
This brachiopod has tiny spine-line projections from the shell. In some brachiopod groups, these spines were really long, longer than the size of the shell itself, perhaps to keep brachiopods from sinking into soft sea bottom mud.
Compared to the brachipods nearer the top of the page, this group has a much narrower hinge line, and are very easy to identify.
You can't see it in the photograph, but the brachiopods above are quite flat, as compared to the more rounded, full-bodies shape of the other brachiopods shown above.
The distinguishing characteristic of this group of brachiopods is the finely striated, (finely ribbed) shell.