Spermacoce diffusa Willd. ex R. & S., Syst. Veg. 3: 531. 1818; H.B.K. Nov. Gen. & Sp. 3: 343. 1819.
The isotype sheet of Mitrocarpum puberulum Benth., based on Rob. Schomburgk 394, at US is conspecific with Mitrocarpus diffusus, but the NY isotype specimen of Mitrocarpum puberulum is conspecific with Borreria ocimoides (Burm.) DC.
The distribution of this taxon appears to be limited to the drainage of the Orinoco River in western Venezuela, where it appears to abound in the rivers of Apure and Barinas states. The collections cited from those states are near the type locality of Spermacoce diffusa. Mitracarpus diffusus has been confused with M. villosus (Sw.) DC. (M. hirtus), but is amply distinct with the following differences: in M. diffusus the hypanthium is glabrous throughout, except where sparsely pubescent on the upper angles, the shorter pair of calyx lobes is shorter, the corolla is smaller (1.2 mm instead of 2 mm long), the pubescence of the stem consists of rigid, subappressed, upcurved, short hairs throughout instead of having elongate, spreading, loose hairs on the angles, the seeds are smaller with a larger central body and shorter broader radii, the pubescence immediately below the flower-heads consists of short, dense, appressed hairs, instead of elongated, loose, descending to spreading hairs, the stipular sheath surrounding the heads is pubescent and shallower (in M. villosus it is 2.5-3 mm high), the corolla is shorter than the longer pair of calyx lobes, the midrib and lateral nerves of the lower leaf surface are more deeply imbedded in the thicker leaf tissue and the hairs shorter and more scabrid on the nerves and surface, whereas in M. villosus the midrib and lateral nerves are more elevated with longer hairs, the fimbrillae at the base of the flowers are less numerous and less conspicuous, the stems are more diffusely branched and spreading, the anthers are shorter and more suborbicular in shape, and the flower stipe is shorter and less conspicuous.