The following species apparently lies within the circumscription of Microdictyon but has not yet been transferred or reduced to synonymy:
Macrodictyon clathratum J. Gray, 1866b: 292–293 (syntype localities: Pulau Tikus, near Bengkulu, Sumatra, Indonesia; Hawaiian Islands).
INDIAN OCEAN DISTRIBUTION: Indonesia (Sumatra).
Note: Macrodictyon clathratum, which was described by J. Gray as the only species in a new genus, was based on two specimens sent to him by Lenormand. The first was a specimen collected by Martens at Pulau Tikus, Sumatra, bearing the manuscript name Microdictyon clathratum Martens and mounted on talc. The second specimen was a large, roughly dried mass collected by Éduard Jardine in the Hawaiian Islands. Setchell (1926b: 151; 1929: 457–458) was of the opinion that J. Gray based his description largely on the Hawaiian collection, which Setchell referred to Microdictyon velleyanum (op.cit.: 561–568, figs. 85–92). Setchell erroneously interpreted M. velleyanum Decaisne (1841: 116) as an available name for the Hawaiian species (a specimen of which was cited by Decaisne) rather than an illegitimate change of epithet when transferring Conferva umbilicata Velley to Microdictyon. Howe (1934: 38) pointed out Setchell's error and proposed the name M. setchellianum for M. velleyanum sensu Setchell. With regard to the Sumatran collection, Setchell (op.cit.: 458) thought that it was not conspecific with M. clathratum Martens, which is clearly representative of Anadyomene sect. Cystodictyon, but rather "a coarse Microdictyon of the M. Velleyanum assemblage (possibly M. pseudohapteron Gepp and Gepp)''. Later in his monograph, Setchell (op.cit.: 548) added M. okamurae Setchell as a possible placement of the Sumatran specimen. Both of these species belong to Setchell's "M. Velleyanum assemblage'', to which Setchell applied J. Gray's generic name at infrageneric level, namely, Microdictyon sect. Macrodictyon (J. Gray) Setchell (1926b: 149; 1929: 548). Depending on lectotypification and the taxonomic placement of the lectotype, Macrodictyon clathratum J. Gray could be the earliest available (and thus epithet-bringing) name for Microdictyon pseudohapteron, M. okamurae, or M. setchellianum.