Clamshell Cloud

One of the magical elements of Tucson summer's is the diversity of cloud formations that roll in from the mountains as monsoon storms build, crash, evaporate.  After a coolish evening set in this week, Mark and I returned to our walk along Mountain Avenue and we saw this cloud.  It looks like a clamshell to me and I am sure a meterologist can give a specific explanation for its shape.  But sometimes, I don't want explanations, just the experience.  What about you?  And do you see something else in this cloudshape?  Imagine.