RIC VII's helmeted-bust descriptions leave a lot to be desired, both in terms of understanding what they are actually referring to, and how to attribute in case of coins not matching the canonical
bust descriptions (particularly an issue for
Rome 318-319 AD, when there is a profusion of helmet varieties seen on the coins).
There are three
types of helmet portrayed on
RIC VII coins:
1) Up to c.318 AD there's a helmet with a protruding cap-like peak, same as we see in
RIC VI. A
RIC VII note on p.348 refers to this as an "archaic" or "Athene" helmet, but it doesn't seem to correspond to any real
type of helmet I've been able to find photos of. It really seems to be a "pseudo-Corinthan" helmet, mirroring the appearance of
Athena's pushed-back Corinthian helmet, while not actually being one.
2) From c.318-319 AD there are varieties of ridge helmet, as seen on the
VLPP coin
type. This is a period correct helmet
type. It is bowl-shaped, constructed from two halves, or four quarters, joined together at the top with a ridge-like seam. There are a number of museum specimens of this helmet
type, such as the ones from Berkasovo and Intercisa.
3) From c.319 AD onwards there's a helmet with what RIC calls a visor, but is probably a fixed upright browband. This is sometimes referred to as an Attic helmet (as seen on later depictions of
Athena, such as on the new-style owls), or could be called a praetorian helmet based on it's depiction on the praetorian relief. This helmet
type is also seen on the
arch of Constantine.
There's a couple of exceptions to the above timeline, but it gives the big picture.
These helmets are usually depicted with some
type of crest - either a tall feather crest, or more often a (typically lower-lying) horsehair one.
Now, RIC gives us three basic helmeted
bust varieties (D2, D6 & D7), which you might expect (if you didn't know better) to correspond to the three
types of helmet... but of course they don't. For some reason RIC lumps the unrelated helmet
types 1) and 3) together as
bust type D2, described just as "helmet", then uses
bust types D6 and D7 to differentiate two varieties of ridge-helmeted
bust.
The D6 and D7 busts are described as:
D6: laureate, helmet
D7: high-crested helmet (bowl-shaped)
As long as you have a coin that only matches one of these descriptions then this is at least clear enough to attribute by, but what if your coin matches both, or neither, or you're just not sure (is it a "high" crest or not ?). It turns out there's enough information in RIC (by way of attributed museum specimens - thanks Lech!) to see how RIC (Patrick Bruun) intended it, which is ultimately to ignore everything else and just look for presence of a
laurel wreath on the helmet (=D6), or not (=D7).
If we assume the canonical D6 and D7 descriptions refer to the most frequently encountered coins, then it appears that "high-crested" really means feather-crested as opposed to horsehair-crested, but given the above you can in fact ignore crest
type altogether and just attribute based on laureate or not. I'm not sure this is the most insightful way to distinguish the helmet varieties, but seeing as the outliers are mostly transitional it doesn't make much difference.
A picture is worth a thousand words, so since RIC doesn't give us one, here's how to attribute
RIC VII helmet
types. The quoted descriptions are RIC's
bust descriptions, and the snarky comments are my own.