Shield types
Petra: Small light shield of Greeks and Macedonians. Possibly the same as 'pelta' or peltarion, which is noted as being both crescent shaped and round. I suspect the 'crescent' shaped may refer to it's concavity. In any case, smaller then the Clipeus, and carried by skirmishing infantry (Peltasts). http://uoregon.edu/~klio/maps/gr/peltast2.jpg is the image provided, by may be made up.
Cetra/
Targe: Small round Celtic shield, w/ center spike. Laminated wood?
Parma: Small round Greco-Roman shield, strongly made, used metal in construction.
Scuta: Large rectangular Roman shield; concave. 2.5' wide by 4' tall. -Early scutari are sometimes oval?
Clipeus: Large round Greek shield; held using strap (balteus)and central handle (umbo). Concave. Wicker base, covered with ox-hide and edged with metal. Used by Homeric heroes, it later shrank in size. Later the balteus and central handle were discarded in favor of a series of ties.
Gallic: Large oval Celtic shield, w/ boss. Probably laminated wood.
Hoplon/Argive: Greek shield built around wood core, using balteus and grips? 15.5 lbs., 30-39" diameter, used in shield wall.
Shields in Phalanx listed as protecting more the person to the left then the person holding the shield; center boss may have been used as a place to 'hook' the elbow, so more then 1/2 of the shield covered the next person in line.