A family of rather small chitons, that have narrowly shaped shells. The valves have no insertion plates and there are about 100-120 species dispersed over the world. They live in rather deep water, even very deep water, and discoveries are permanently ongoing. A few only live intertidal.
For a list of genera and species:
http://www.marinespecies.org/aphia.php?p=taxdetails&id=59