Black Medick

I found this growing out of the top of a wall and realised I'd forgotten how to distinguish Black Medick, Medicago lupulina, from Hop Trefoil and Lesser Trefoil. The answer is that Black Medick is hairier, has spirally coiled black fruits, and the leaflets are mucronate. 
The word 'Medick' comes, according to Geoffrey Grigson, from herba Medica which means the Median or Persian herb. Strabo had it originating in the country of the Medes. Dioscorides' description shows that it was originally referring to M. sativa, or Lucerne, which was widely grown as a fodder plant in the Middle East. Lupulina derives from 'wolf-like', referring to its similarity to the hop or 'willow-wolf'.