Field sow-thistle

Field sow-thistle.  Consider me schooled: I thought that everything that looked vaguely like a dandelion must be a dandelion.  Not so.  There are not only several kinds of dandelions other than the common one, but there are also some other flowers that look similar.  Sow-thistles have dandelion-like flowers, but have extremely jagged leaves that look very different:

Dandelion for comparison: notice how the leaves' serrations point back toward the plant.

This is a field sow-thistle, not a common sow-thistle, because rather than being smooth the stem is covered with tiny prickly hairs.

Growing out of the stones comprising the south staircase up to the western 116th street exit from Morningside Park, Manhattan.