This word is a play on the many morphisms in mathematics, but cryptomorphism is only very distantly related to isomorphism, homomorphism, or morphisms.
They are an extinct group of ammonoid, which are shelled cephalopods related to squids, belemnites, octopodes, and cuttlefish, and more distantly to the nautiloids.
Today, though most palaeobotanists regard them as being only distantly related to ferns and these names as misleading, but the names have nevertheless stuck.