Here is a photography of a sample of shale from the Burgess Shale. If you look closely, you will see it is full of fragments of fossils. Most are too small to identify.

There are three larger fragments near the center of the image. On the large dark fragment, we can clearly see the ribs. This is typical of a Brachiopod or a Mollusc than an Arthropod, which includes the famous Class of Trilobite. Brachiopods and Molluscs have both existed since the Cambrian and are with us today. The smaller black fragment is similar to the larger fragment. My guess is that these are both Brachiopods.

Above and to the right of the dark fragments, is a light brown fragment. It also has clearly defined ribs. However, the wider triangular rib in the center which leads back the hinge of the creature is more typical of a Mollusc.

What Class, Order, Family, and Genus these creatures are, is far beyond my paleontological ability.