Detail of pronotum, flash set to give a bit more light from one side in order to model features a bit. Quoting from Ciegler, Ground Beetles of South Carolina, p. 27 (1):

4(1) Pronotum with setigerous puncture near hind angle; lateral raised bead of pronotum sharply defined inwardly for its entire length---> C. sayi

Pronotum without basal setigerous puncture but with usual lateral one at midline; lateral bead obscure, at least basally ---> 5 (C. externum, C. calidum)

I think I see a "basal setigerous puncture" here, and it looks to me like the pronotum does have a lateral bead all the way along the sides to the base, so this should be C. sayi. Compare to the pronotum of another individual believed to be of that species:



It would be desirable to have a comparion photo of one of the other species at this point in the key. C. calidum has very similar markings to C. sayi, but is apparently rare or not present in my part of North Carolina. C. externum, however, I've seen:



I think (!) I can see what the key is talking about--crossing my fingers, as usual. (C. externum also has rather different coloration from C. sayi, as is obvious here.)