The corolla in this taxon is smaller than that of M. scabrellus, varying from 1.2-2 mm long, whereas in M. scabrellus it is usually 2.5-3, rarely only 2 mm long. Bremekamp gives the corolla length as only 0.8 mm (in Pulle, FI. Suriname 4(1): 294. 1934), but all the Suriname specimens I have examined, including Pulle 542 (cited by Bremekamp), have longer corollas. As stated in the key to species, the corollas of M. microspermus are shorter than or barely equal the longest pair of calyx lobes, whereas in M. scabrellus the corolla is at least twice as long as the calyx lobes.
Schumann states “Guiana Anglica” as the locality for Robert Schomburgk’s 856 collection from Rio Branco. This locality is probably in Brazil and the collection of Ule 8323 a near topotype. Since Schomburgk 856 in part, is likewise, the type collection, of Mitracarpus scabrellus Benth., it is necessary to indicate that the smaller-flowered specimen with corollas 1.2-2 mm long is to be taken as the type of M. microspermus Schum., as cited by Schumann, whereas the larger-flowered specimen with corollas 2.5-3 mm long, deposited at K and US, are to be taken as the type collection of M. scabrellus. Standleyi report of M. microspermus for Venezuela, based on Gaillard 208 (P), is in error, as an examination of the specimen proves it to belong to M. parvulus Schum.