I won't list the taxonomic details from the book, as those change often and the book was published in the late 80s. For any interested, the description given is:
Shell is oval in outline, pedicle and brachial valves convex, pedicle valve with broad shallow sulcus and brachial valve with corresponding broad fold. Ornament of fine growth lamellae.
Dielasma was an epifaunal suspension feeder, meaning that it lived on the sea floor (as opposed to within it) and sifted out its food from the water (using an appendage known as a lophophore). They are the ultimate couch potatoes of the sea. They had a thick, fully functional pedicle, which is the bit which attaches to the substrate so that the brachiopod stays put (also indicating that it is epifaunal; without it would suggest it was part of the infauna). The pedicle may have divided into rootlets to give an even stronger hold. Dielasma are often found as nests and with a wide range in size.
It would be interesting to find Dielasma attached to other organisms, such as a bryozoan reef, as this sort of thing often happened. Many fossils have little brachiopods attached to them, particularly sponges, even way back in the Cambrian.