Although the following pictures have many different shapes, they are all slices through the same type of fossil. These fossils all have thick walls relative to the thin shells of fossils, 1, 2, and 3.
The first two images are slices through the fossil, from top to bottom, as if it was cut in half. The next four images are slices across the fossil, from side to side. In some cases, or at some part of this fossil, it is closed and circular or O-shaped, in others it is partly open or C-shaped.
The next picture is a slice through the fossil, from bottom to top, cut at an angle such that you can see that part of the fossil is thick-walled and closed or O-shaped, and part is open-walled and C-shaped. You can also see that the outer walls of this fossil can bulge in and out. In fact, the organisms that make this type of fossil come in many shapes and sizes. Can you guess the original shape of these fossils from the pictures?
To see the three-dimensional shape and learn what type of animals made these fossils go to the fact sheet. http://www.uky.edu/KGS/education/YoungFossils-11.pdf
Leave this website to see what modern animals with this shape look like. http://www.ucmp.berkeley.edu/porifera/poriferalh.html