I've never been exactly confident on my identification of this common lawn plant as Black Medic(Medicago lupulina) since my guide book doesn't give a good sense of its size and there is another common flower that is very similar but with flowers that are 2 to 3 times larger. So in the above photo notice the tiny syrphid fly; those little guys are a bit smaller than mosquitoes. Around here, this is a "weed" in lawns that thrives where the grass doesn't do well, like by the sidewalk in my lawn. As it's green and can be mown, it is welcome in my lawn any time.
I finally took the time to go through and figure out what it was using the web. There are 3 species of hop clover that are very similar to black medic. Since it isn't the right time of year to see the seedpods the identifying feature that nailed it for me was the tiny spike at the end of each leaflet on Black Medic. A couple of websites pointed this out as a way to distinguish black medic from its look-alikes. The look-alike I find around here (that really doesn't look very similar when you compare the real thing side by side) is Hop Clover(Trifolium aureum). Below are close-ups of the two flowers