Scott and Mike,
The problem is that Tantilla gracilis and Sonora semiannulata both have 6 supralabials, 15 dorsal scale rows, and a divided anal plate. The key between the species is the presence (Sonora) or absence (T. gracilis) of that loreal scale between the preocular and the nasal scale. It looks like Tennant's key uses the loreal to distinguish between the species as well (neither species has a row of subocular scales).
Michael was kind enough to forward me a couple of other pictures that certainly show a beautiful red belly on this beast - an interesting clue leaning towards gracilis as the ID. Unfortunately, the head shot isn't clear enough to show the scale pattern on the head.
I really appreciate the opportunity to look at the other pics of the animal - thanks!!
travis
austin, tx