Complex sentence: Their name is Celtic in form and many writers believe that the Teutones really were Celts, perhaps stemming from a branch of the Helvetii, however, a people of this name are mentioned by the early traveller, Pytheas, as inhabitants of the northern ocean coasts.
Split into simpler sentences: Their name is Celtic in form and many writers believe that the Teutones were really Celts, perhaps stemming from a branch of the Helvetii. However, a people of this name are mentioned by the early traveller Pytheas, as inhabitants of the northern ocean coasts.