Complex sentence: The idea of a dog for a handle is thought to come from Greek or Etruscan culture; a dog is repeated twice on the lid, whereas the spout ornamented by a duck appears to be a wholly Celtic creation.
Split into simpler sentences: The idea of a dog for a handle is thought to come from Greek or Etruscan culture and it is repeated twice on the lid. Whereas the spout which is ornamented by a duck appears to be a Celtic creation.