by Bob Harms 

Key in Diggs et al. 1999, Illustrated flora of North Central Texas (BRIT, p. 758):
Characters with significant overlapp for H. drummondii & H. reverchonii var. serpyllifolia are highlighted in red.

3. Plants with aroma of peppermint; leaves usually more than 3 times as long as wide, bright green; upper and lower calyx teeth usually strongly convergent (± completely closing the calyx opening at maturity); corollas 7— 11 mm long, blue; annuals or herbaceous to woody–based perennials averaging < 25 cm tall
[in text calyx length is given as 5—6 mm long]

… H. drummondii

3. Plants with aroma of camphor or lemon; leaves usually less than 3 times as long as wide, gray green or dark green; upper and lower calyx teeth incompletely closing opening; corollas 8—15 mm long, white or lavender; woody–based perennials usually averaging > 25 cm tall

… H. reverchonii

then for H. reverchonii:

1. Plants lemon–scented; leaves gray green; calyces 6—7 mm long; corollas 10—15 mm long, the tube conspicuously dilated upward

… var. reverchonii

1. Plants camphor–scented; leaves dark green; calyces 5—6 mm long; corollas 8—10 mm long. the tube only slightly dilated

… var. serpyllifolium

[sic., should be var. serpyllifolia; hedeoma is feminine.]

This key, largely based on R.S. Irving's 1980 key, rightly gives greater prominence to scent differences. Note also the number of ‘usually’ hedges. Although it is somewhat simpler, it errs in adding flower color as a distinctive characteristic (as well as repeating leaf color from Irving's key).