Don't be decieved by the anvil like shape, heavy looking precipitation and icy look of this cloud! This is far from the size and power of an actual Cumulonimbus. But it is interesting in that it fits every criteria of a Cumulonimbus capillatus incus, except the bit about considerable vertical extent for the genus. I'm quite sure that this guy didn't raise its head much above 2 km AGL.

I suppose it could be classified as a Cumulus of some sort, but it really doesn't fit well under any genus. Our systems of classification are just something that we made up. Clouds are under no obligation to conform to them. The same is true for everything else in the universe as well.