Triarii

[3] The first class comprised the richest soldiers in the legion who were equipped with spears, breastplates and large shields, like heavy Greek hoplites.

After a time, engagements with the Samnites and Gauls appear to have taught the Romans the importance of flexibility and the inadequacy of the phalanx on the rough, hilly ground of central Italy.

They also carried swords, or gladii, about 84 centimetres (29 inches) long, in case the spear broke or the enemy drew too close.

[6] They fought as hoplites, usually carrying clipei, large round Greek shields, and wearing bronze helmets, often with a number of feathers fixed onto the top to increase stature.

[9] Their equipment and role was very similar to the previous system, except they now carried scuta, large rectangular shields that offered a greater degree of protection than the old round clipeus.

[11][12] Pitched battles were conducted in a similar fashion: the velites would gather at the front and fling javelins to cover the advance of the hastati.

At the Great Plains, Scipio, the Roman general, formed his men up in the usual manner, but once the hastati had begun to engage the enemy, he used his principes and triarii as a flanking force, routing the opposing Carthaginians.

Once the surviving elephants had been routed, Scipio formed his men into a long line with his triarii and principes in the centre and hastati on the flanks, ready to engage the Carthaginian infantry.

[15] With the putative military reforms of Gaius Marius in 107 BC, implemented to combat a shortage of manpower due to wars against Jugurtha in Africa and Germanic tribes to the north, the different classes of units were scrapped entirely.