But Andrew-

There is always one exception to the rule?   Does one fertile mule mean that the majority of mules are fertile, which would
certainly point towards the parent species hybridizng to be
separate species.

Returning, to shells, I have always thought that the future of speciation lies in the examination of the animals - the surface of which we are only "scratching."   I am thinking of a recent description of a new species based entirely on the shell, without ever seeing even one of the animals.    Let us hope that more and more research can be done on the snimals, without decimating the populations.

Reactions?

Curious Linda