The establishment of the genus Microdictyon by Decaisne (1841: 115) was prompted by the receipt of a collection made in the Red Sea near ``Djedda'' [Jeddah, Saudi Arabia] by Paolo [Paul] Emilio [Emil] Botta. [During the period 1826--1839, Botta (1802--1870), who was born in Italy but had become a French citizen, retraced the route of the ill-fated explorer Pehr Forsskål (1732--1763).] Representatives of the genus had been described previously, but had been assigned to Conferva, Hydrodictyon, and Anadyomene. Decaisne ruled out Hydrodictyon as an appropriate genus for Botta's material on the basis of differences in habit, habitat, and morphology. In Hydrodictyon the thallus floats freely in fresh water, while Botta's alga was attached to submarine rocks. Although life histories were unknown in 1841, the formation of daughter nets within the parent net of Hydrodictyon was vaguely perceived by Decaisne, who contrasted the resulting double wall with the single wall observed in Botta's alga. Anadyomene, which resembled Botta's alga in habit and habitat, was distinguished by its precise pattern of reticulation.
Although the concept of Microdictyon seemed clear in Decaisne's mind, the delineation of species within the genus was uncertain. He described Botta's alga as a new species, M. agardhianum, but then introduced confusion by conjecturing whether previously described taxa might belong to it. Anadyomene calodictyon Montagne (1841 [1839--1841]: 180, pl. 8: fig. 1), from the Canary Islands, was considered a possible synonym. Decaisne recognized that Conferva umbilicata, which had been described from Sydney Harbour, Australia, by Velley (1800: 169, pl. 7) and transferred to Hydrodictyon by C. Agardh (1824: 85), was a distinct species of his new genus, but puzzled over H. umbilicatum var. tenuius C. Agardh (1824: 85). This taxon was based on a specimen from Cádiz, Spain, but was invalidly published since no description was given beyond the adjectival epithet. Decaisne stated that he believed that he was able to report Botta's alga to Agardh's variety (hence the epithet agardhianum), in which case there would be only two species in Microdictyon, namely, M. velleyanum Decaisne (an illegitimate change of epithet when transferring Conferva umbilicata Velley to Microdictyon) and M. agardhianum Decaisne, the latter including collections from the Canary Islands, Cádiz, and the Red Sea. He admitted, however, that H. umbilicatum var. tenuius could be representative of a distinct species, for which he suggested the name M. tenuius. This binomial was intended to be an elevation to specific rank of H. umbilicatum var. tenuius, but since the intended basionym was invalidly published and Decaisne did not provide a description, M. tenuius was also invalidly published. The three species that would result from this taxonomic decision were M. velleyanum, M. agardhianum, and M. tenuius. These three species were recognized by Decaisne in his thesis (Decaisne, 1842a: 327).
Regarding Decaisne's confusion in relating the Red Sea material to previously published taxa, Setchell (1929: 480) commented, ``While I am by no means certain that I fully comprehend the mind of Decaisne in this matter, it seems fairly certain that he included under M. Agardhianum the Anadyomene calodictyon Mont. from the Canary Islands''.
Endlicher (1843: 14), misinterpreting Decaisne's intentions, placed Hydrodictyon umbilicatum var. tenuius (incorrectly cited as `tenuis') in the synonymy of M. agardhianum, retained M. velleyanum, and listed Anadyomene calodictyon as the third species under the illegitimate name M. montagneanum Endlicher. Kützing (1849: 512) restored Montagne's epithet by publishing the combination M. calodictyon (Montagne) Kützing while merging the remaining species with M. agardhianum.
J. Gray (1866a: 66--69), who was the first worker to monograph the genus and its close relatives, made it clear that M. agardhianum was a superfluous name for M. calodictyon. He referred Botta's material from the Red Sea to M. tenuius Decaisne and validated that name by paraphrasing Decaisne's description. Although Gray cited Hydrodictyon umbilicatum var. tenuius in the protologue of M. tenuius, he did not list Cádiz as a locality. He recognized three other species, namely, M. velleyanum, M. montagnei Harvey ex J. Gray (a new species from Tonga), and M. kraussii J. Gray (a new species from Natal). In a postscript to his monograph, J. Gray (1866b: 291) changed the name M. tenuius to M. tenue, probably in an attempt to correct a perceived error in grammar when in fact no error existed. The epithets tenue and tenuius are distinct, the former being a positive adjective (``tenuous''), the latter a comparative adjective (``more tenuous''). The comparative adjective tenuior is correct with masculine and feminine generic names, tenuius with neuter generic names. Setchell (1929: 479, 485), however, treated M. tenue separately from M. tenuius, citing it as a synonym of M. agardhianum.
The binomial Microdictyon tenuius is usually ascribed to (C. Agardh) Decaisne, but as explained above, the intended basionym (Hydrodictyon umbilicatum var. tenuius C. Agardh) was invalidly published and Decaisne provided no description. The correct authorship is Decaisne ex J. Gray. Although the epithet suggests that it is based on the collection from Cádiz cited by C. Agardh, the type is unequivocally the Botta collection. According to Setchell (1925: 105), no representative of this collection could be found in the Decaisne herbarium at Paris (PC), but a specimen sent to Lenormand by Decaisne was found at Caen (CN) (J. Gray, 1866b: 291, 292; Setchell, 1929: 463, 479--485, figs. 1--5). Although it is clear that M. agardhianum is a superfluous name for Anadyomene calodictyon, it is not clear whether it has the same type as that species or whether Decaisne ``designated or definitely indicated'' a different type (see Art. 7.5). This decision determines whether M. agardhianum is to be treated as a nomenclatural synonym of M. tenuius or as a name misapplied to records of this species. More importantly, it determines whether the correct name for the type species is M. tenuius or M. calodictyon. To preclude continued uncertainty, I shall submit a proposal to conserve Microdictyon with the Botta collection as type.
Subsequent workers have not interpreted M. tenuius correctly. J. Agardh (1894a: 105--107) erroneously ascribed the name to C. Agardh and applied it to a species comprising all elements of the genus known to Decaisne except M. velleyanum. He recognized four geographical forms. Anadyomene calodictyon was included as a synonym of the nomenclaturally typical form (forma atlantica), thus rendering J. Agardh's use of the specific name M. tenuius incorrect. The Red Sea entity was assimilated as M. tenuius forma erythraea. The other two forms were based on collections not seen by Decaisne, forma mediterranea being based on Zanardini's treatment of M. umbilicatum from the Adriatic, while collections from Australia and Tonga were described as forma australis.
Setchell (1929: 486--490, figs. 6--9), taking a different tack, restricted M. tenuius, which he ascribed to (C. Agardh) Decaisne, to Cádiz, the intended type locality of Hydrodictyon umbilicatum var. tenuius. His description was based primarily on the intended type specimen (no. 16201 in the Agardh Herbarium at LD), supplemented by observations of topotype specimens collected by Børgesen.
At the species level, the imbroglio discussed above would be made moot if future taxonomic studies were to suggest that the Red Sea populations of the genus should be assigned to either M. calodictyon or M. umbilicatum. The question regarding the type of the generic name, however, is not affected by changes in infrageneric classification.