I'll leave to others to comment on the habits of Crepidula fornicata, which
were indeed accurately portrayed in the article reproduced by Dominic
Rawlingson Plant. I will tell you about Crepidula princeps instead.
 
Crepidula princeps (princeps = "chief", later "prince") is a large slipper
snail that lived on the California coast during the Pliocene Epoch. Its
habits must have been similar to those of C. fornicata, because specimens
are preserved in curving stacks of 6 or more snails. In fact, this was one
of the first fossils that I encountered on a field trip as an
undergraduate. The professor, Norm Silberling, took us to San Gregorio
beach in central California to show us sedimentary rocks.
 
Many of the best outcrops in California are seacliffs, which can be
distracting due to their natural beauty. In fact, Norm later told me that
he nearly flunked his first geology course because he paid more attention
to watching birds than rocks.
 
But to return to the story, I was thrilled to find ammonites in the cliff
at San Gregorio, and pointed them out to the professor. He didn't know what
they were, but he did remark that the rock was Pliocene. Even I knew that
the last ammonite died more than 60 million years before that. I would have
been better off calling them nautiloids, which they resembled more closely
than ammonites, but I wasn't very knowledgeable about mollusks then. They
remained a mystery that day. I later learned that they were neither
nautiloids nor ammonites, but stacks of the gregarious slipper snail,
Crepidula princeps, and a rare example of fossilized reproduction.
 
Thanks for bringing back the memory, Dominic. I wish I could be there now
on that cool, windy beach, building sand castles and taking a long walk on
the beach below the tall, yellowish, fossiliferous cliffs.
 
Andrew K. Rindsberg
Geological Survey of Alabama