Seratonia: why is it not spelled with an s?

Because the sound is younger than the spelling. The Greek word behind the name is keras, horn, for the curved pod, and it began with a hard k. Latin wrote Greek kappa as c and kept the hard sound. Then, in the mouths of Latin speakers and later in French, c before a front vowel drifted towards s. English inherited the drifted sound and the undrifted letter. What makes this worth noticing is that the same Greek horn reached English by two other routes and kept its k both times. Rhinoceros is nose horn, and keratin, coined by nineteenth century chemists straight from the Greek, is the horn substance in your fingernails. Those words were built by people reading Greek. Ceratonia came the slow way, through Latin and centuries of speech, and arrived with a c that no longer says what it once did.