Guest, Bidens and desmodium are very different. Bidens species are in the aster family.   Rayed flowers, usually yellow. The seeds are usually black or striped black with two prongs on one end. Desmodium species are unmistakably legumes. Blooms are usually violet to pink and typical of pea family blooms. Leaves consist of three leaflets. They form long or longish seed pods as with other peas, but with "velcro" on the exterior, and the pods may be segmented between seeds to the extent that if you brush against them with a pants leg, part of the pod will break off and cling.

A number of different species and families of plants have seed pods that are rounded, spiny burrs. Many, but certainly not all by any stretch, are in the aster family, (a huge family, btw.) Sedges and grasses are the other two most common families of plants that produce spiny, rounded seed capsules.