Thanks for your suggestion. Have just received a message from Dr. Arthur Shapiro at UC Davis, who says: "It's an old female P. campestris (Field Crescent). The antennal club is half brown (basal) and half yellow (distal). The median band is often paler than the rest of the ground color, and on old specimens may fade to almost white (as here). Note that much of the hair has worn off the back of the thorax, another indicator of aging; I'd guess she was 2-3 weeks old. There is nothing at all unusual about her in the context of long series of the bug! Half the trick in a field guide is convincing readers that a certain amount of variation is to be expected.

BugGuide follows the species names on All-leps, and thus this will be placed in Field Crescent, _P. pulchella_ , since _P. campestris_ is not a species name recognized there.

Fine with me - I'll adjust my records. Appears the name has changed - two older field guides in my collection show "campestris" but the two newer ones show pulchella! Many thanks to all for your help!

So is this the identification? My book is in Vermont, but the pictures I have for Northern and Pearl Crescent don't look like this.
Is it a female? The club is yellow and brown as indicated in the first comment.

Donna, I can see why you think this is similar to your mystery butterfly. But this truly is a western species, so I would be very surprised if yours turned out to be a Field Crescent. I don't know anything about eastern butterflies, but based only on Kaufmans field Guide to Butterflies, I would never be able to relate your picture to either Northern or Pearl Crescen, either - though granted they do occur in the right part of the country. Have you considered Phaon Crescent (Phyciodes phaon)? It's primarily southeastern, but the range map shows it going almost to New England, and the picture is closer to the picture of your mystery butterfly. Haven't compared size at all. Good luck!

A search on both Field Crescent and P. campestris in the Guide returned P. pulchella as the scientific name of this species. I was going to move this to the guide since it has an ID now, but not if the Guide won't accept the species name.

I'll bring this up in the forums and see what consensus comes out...