If you said this was a Northwestern Fritillary because of the lighter submarginal/postmedian band, note that it still has a red tint to it, more pink than the yellow-beige of the Northwestern Fritillary photos I've shown. Also, the upper three cream spots in the center band are oval-shaped, not pointed (compare with #D below). All other species in the test have silvered spots, except the Mormon Fritillary which can also be both silvered or unsilvered, but the brick-red color identifies this as Hydaspe.
This is an average-looking Zerene: rounded silver spots, beige VHW ground color with reddish-chocolate overtones. It is a little more reddish than some specimens, but not red enough to be Hydaspe, plus it has silvered spots. It is too dark and the silver spots are too rounded to be Callippe. It may be confused with Coronis because the silver spots have a little more black around them than some of the other Zerene photos I showed, but the complete lack of olive/green scaling on the VHW identifies it as Zerene, not Coronis.
C - Great Spangled Fritillary Argynnis cybele
The size of this specimen (2.5 inches) would give it away as Great Spangled if you saw it in person. It might have been confused with the Aphrodite Fritillary but as I mentioned before, I don't have any photos of those. The reduced size of the black spots on the dorsal side, and of the silver spots on the ventral side, give this away as a Great Spangled Fritillary.
Unsilvered spots, so it has to be either Northwestern, Hydaspe, or Mormon. The discal area is too dark to be a Mormon Fritillary, and not red enough to be Hydaspe. In addition to that, the submarginal/postmedian band is yellow-beige, not pinkish like Hydaspe, and the center band of cream spots are much more pointed than Hydaspe's spots.
F - Hydaspe Fritillary Argynnis hydaspe
I picked this specimen because some of the cream spots are a little more elongated than average Hydaspe, and the submarginal band is a little lighter because this is a slightly faded/older individual. As I explained for specimen #A above, the VHW ground color is brick-red, not chocolate-brown as in the Northwestern Fritillary, and the submarginal band still has a pinkish-tint to it, not the yellow-beige seen in the Northwestern.
This is the other tricky specimen I was referring to, and the only one here that I am not entirely confident about, so if you thought it was a Callippe or Zerene, you might actually be correct, but I will explain why I believe it is Coronis. It is a female, which makes it even more difficult to determine, because the standard markings that make the males easier to identify are a little different. This is one of those cases where you can't really make the determination based on a single aspect, such as color or spot shape, but taken together they point towards one species. Please feel free to argue with me on my following explanation, it might help one of us come to a stronger conclusion!
1. Overall size. You can't tell from the photo, but this is a larger butterfly (2.25 inches). Female fritillaries tend to be larger than males anyway, and 2.25 inches is not uncommon for Zerene and Callippe, but a larger size often points towards Coronis.
2. Separating it from Callippe. The silver spots are a little more elongated and slightly pointed than the average Coronis, which would seem to point more towards Callippe (also read my explanation for #L). However, the spots do not fill the VHW quite as much as Callippe usually do, and there is no greenish dusting of scales over the whole surface. The base color has a little bit of a greenish tint common to Coronis and similar to Callippe, but the markings are still sharp, not entirely clouded over like Callippe often are (see #H, male Callippe in the photo below). If you are saying "now wait a minute, what about that first female Callippe in the last post?" (see here), yes I know it looks an awful lot like this one, and I'm not 100% sure on that identification either. However, notice that the ground color in that specimen is much more even-toned, not darker in the discal area like in this specimen above, and the silver spots (of the "Callippe") aren't quite as sharply framed as in this one (#G, "Coronis").
3. Separating it from Zerene. The VHW ground color is more yellowish/greenish-brown and chocolate, with almost no hint of the rust-coloring you see in Zerene.
On a side-note, this is actually a slightly-aberrant specimen; the tips of the forewings are not missing scales as it appears, the scales are there but lack normal coloring, while the rest of the butterfly is normally-marked.
This is a case where it would be a dead give-away if you saw this specimen in person, because it is smaller than all the others shown here, with a wingspan just over 1.5 inches. I knew that without a size reference you would have to depend on other points of identification, so I chose a specimen that had a lighter VHW ground color and silvered spots, similar to Zerene. The difference is that this does not have the reddish-tint in the VHW color you see in Zerene (#J, below), and the black markings on the dorsal surface are much smaller than Zerene. Although the spots are silvered, there is still a cloudy appearance that you will not see in Zerene, and this cloudiness also eliminates any defined framing of the silver spots that you would see in Coronis.
This is another standard-looking Zerene, similar to #B. The silver spots are not large or elongated enough to be easily confused with Callippe, and the VHW color has no trace of green that would be seen in Callippe and Coronis.
Fritillary Frenzy post, and hopefully you'll start to see the subtle differences in color (Coronis v. Zerene) and spot shape/size (Callippe v. Zerene and Coronis).