"David E. Ross" wrote
I think this is arvensis.
Actually its leaf form is more like the American pennyroyal, Hedeoma
pulegioides. But I wonder if the same compound that's bad is also in
this,
as well as the American.
Here's what the plant normally looks like, maybe someone can confirm/deny
my
ID:
http://webpages.charter.net/slyrp/Pl...20arvensis.JPG

The leaves are I guess at most 25mm long.


It has the square stem and opposing leaves of a mint. However, the
flowers seem to be radially symmetric while mints have flowers that are
only bilaterally symmetric.

That's the problem with trying to photograph a 1.5mm flower (with a
10-year-old digital camera). With a 16x loupe I saw that the petal markings
are bilaterally symmetrical. A page I found on a similar, if not the same
plant:

http://www.wildflower.org/plants/res...id_plant=mear4
"The 4-lobed and nearly symmetrical clusters of flowers along the stem
distinguish this so-called true mint from many others that have flowers in
slender spikes at the stem tips or in upper axils."