I've featured Strophodonta shells from Sylvania before on my blog, in a post about the first fossils I bought, but I thought it would be useful to do a "turn-around" to show what it looks like from different angles.
Here are three specimens that show the variation in size and shape.
This is the largest specimen on the left side of the above picture. Note that it has a very deep concave shape. This is not limited to shells this size and as such could be a variation in growth or perhaps a species or subspecies.
A smaller specimen that is on the lower right of the first picture. Less concave that the previous specimen but greater hinge width when compared to shell length.
I picked this specimen off the piles specifically because the Pyrite replacement of it's shell had not completely oxidized and also because it afforded me a cut away view inside the shell. Most of the Strophodonta shells I collected loose are whole or partially crushed so that you can't get a good idea of the internal space within the shell.
Many Strophomenids, like Strophodonta, did not have hard parts to support it's Lophopores (feeding organs) like Atrypids or Spiriferids. As such there was no need to build a shell to store them.
This is pure conjecture but judging from the narrow space that can be seen in the above side view, the animal inflated it's body and organs with water when it opened it's shell. When they sensed a threat or otherwise closed, they would eject the water and their soft bodies would once again fit within the narrow confines of the shell. Or perhaps they pushed their lophopores out into the current to catch food particles.
Here are two separate valves (non associated) that I've arranged to illustrate what the animal would have looked like when opening it's shell to feed.
Looking at the exterior of the Brachial valve so you can see how the valves may have once fit together (were they from the same animal).
These specimens were collected from the Silica Shale dumps in Sylvania, Ohio in 2004.