The alpha-numeric designations below are from RIC VII.
(i) Plain busts
(A) bare headed busts
|A1 bare headed, draped||A2 bare headed, draped, and cuirassed seen from back (from behind)||A3 bare headed, draped, and cuirassed facing||A4 bare headed, draped, and cuirassed (from the front)|
A5 bare headed, cuirassed
A6 mantled, waved hair
(B) laureate busts
|B1 laureate head||B2 laureate looking upwards||B3 laureate, draped, and cuirassed seen from behind||B4 laureate, draped, and cuirassed (from the front)|
|B5 cuirassed||B6 wearing trabea|
(C) radiate busts
|C1 radiate head||C2 radiate, draped and cuirassed seen from behind||C3 radiate, draped, and cuirassed (from the front)||C4 radiate and cuirassed|
(D) helmeted busts
|D1 helmeted, draped, and cuirassed||D2 helmeted and cuirassed||D3 VRBS ROMA, imperial cloak||D4 VRBS ROMA, helmet with plume|
|D5 helmeted, laureate, draped, and cuirassed1||D6 helmeted, laureate, and cuirassed||D7 wearing high crested helmet and cuirassed1|
(E) diademed busts
|E1 plain diademed||E2 rosette diademed||E3 pearl diademed||E4 plain diademed looking upwards|
|E5 rosette diademed, looking upwards||E6 pearl diademed, looking upwards||E7 plain diademed, draped, and cuirassed||E8 rosette diademed, draped, and cuirassed2|
|E9 pearl diademed, draped, and cuirassed|
(F) veiled busts
1. Also, holding spear and shield (we could not find plain bust examples).
2. This type with laurel leaves between the rosettes is sometimes called "laurel and rosette diademed."
RIC VII Bust types on Constantine the Great coins.com - http://www.constantinethegreatcoins.com/busts/
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Bust. - This term is derived from the Italian Busto and the French Buste. It is applied to representations of people that do not extend below the waist.
One of the most ancient ways of representing gods and heroes, under human features, was that of showing only their heads.
The invention of busts is one that dates from a much later epoch. These show the head and shoulders, and a small |part| of the chest; at other times the head with the whole chest; a rarely, the full upper half of the body.
The Romans had a custom of placing busts of emperors and other great people on their coins, which helps us attribute the coins much more easily today.