Is this a fossil we found in a store?
You have a nice stromatolite fossil.

A stromatolites are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. stromatolite fossils can be some of the oldest fossils found on earth. Living stromatolites can still be found today.
 (Wikipedia).

This pattern on the underside of a stromatolite fossil is very characteristic.


Cretaceous age stromatolite fossils from Morocco are commonly found for sale online. (Esty)