The Poundstone Clypeus ponti

The fossil Clypeus ponti is the so-called "poundstone." This term was used for this species because during Medieval times in western Europe, people would use these fossils as counterweights for measuring one pound of butter or of cheese (i.e., a single specimen = one pound). 

The species is of Middle Jurassic age (Bajocian Stage = 170 million years old). This specimen shown above is 7 cm (2.75 in.) in diameter, and the photograph shows its top (dorsal = aboral) side. The specimen is from France.

There are several very informative articles on the web that discuss the history of the usage of this fossil, but, so far, all the ones I have read refer incorrectly to this species as a "sea urchin." Clypeus ponti does belong to phylum Echinodermata, as do sea urchins, which are regular echinoids. Clypeus ponti, however, belongs to the irregular echinoids. Unlike sea urchins, which live on the ocean floor and have long spine, irregular echinoids (like sand dollars, etc.) burrow into the sea floor and have very short spines.

This is the same specimen illustrated above but shows the bottom (ventral or oral) side. Notice the five-ray symmetry of its feeding grooves

Again, this is the same specimen, but the view is of the front end with its depressed central area. I thank the Natural History Museum of Los Angeles County for allowing me to photograph this specimen of C. ponti.