<P> The primary evidence is scattered archaeological finds, which are often damaged or incomplete . There are similarities of form and function between them . </P> <P> H. Russell Robinson in his book The Armour of Imperial Rome, published in 1975, classified into broad divisions the various forms of helmets that were found . He classified four main types of helmets for heavy infantry (with subcategories named with letters) and 30 different types of cheek guards . </P> <P> Helmets used by gladiators were quite different from military versions . </P> <Ul> <Li> Montefortino helmet (4th century BC - 1st century AD) </Li> <Li> Coolus helmet (3rd century BC - at least 79 AD) </Li> <Li> Imperial Gallic helmet (late 1st century BC - early 2nd century AD) </Li> <Li> Imperial Italic helmet (late 1st BC - early 3rd century AD) </Li> <Li> Ridge helmet, first depicted on coins of 4th - century AD emperor Constantine I . </Li> </Ul>

Why do spartan helmets have a mohawk of hair on the top