Mussel of the

Page last updated
1 December 2021

Mussel of the Month

The December 2021 Mussel of the Month is Ptychobranchus fasciolaris. Ptychobranchus is a genus of six species from eastern North America.

ANSP 20253. Kentucky River (type)

We have shined our spotlight on the genus Ptychobranchus before, but that was years ago. Back then, we took the opportunity to link to Chris Barnhart’s excellent photos of the bizarre glochidial conglutinates that mimic insect larvae. Incredible!

This month, we will use Ptychobranchus fasciolaris to highlight our community’s nomenclatural delusion that primary type specimens exist for the species described by Rafinesque. According to currently accepted lore (e.g., Williams et al., 2008), the valves depicted here for P. fasciolaris represent the lectotype selected by Johnson & Baker (1973). However, as soon as we start tugging on this thread, the sweater of taxonomic respectability unravels.

For those of you that have made it this long without having to busy yourself with the definition of a lectotype — first of all, congratulations! — here is what it says in International Code of Zoological Nomenclature, Article 74.1: “A lectotype may be designated from syntypes to become the unique bearer of the name of a nominal species-group taxon…” In other words, if there is more than one specimen in the original type series, then a subsequent author (in this case, Johnson & Baker) can select one to be the primary type, and any remaining specimens in the type series would become paralectotypes.

The thing is, Rafinesque (1820) did not point to any specific specimens to be his type series in the original publication, although he did imply that he had seen more than one (e.g., “Espèce assez commune”). What made Johnson & Baker (1973) believe that this specimen was part of the type series? Because Vanatta (1915) had reported that these particular valves had been a part of the Poulson collection at the Academy of Natural Sciences of Drexel University in Philadelphia, and Poulson (1832) had said he got the shells from Rafinesque. Actually, what Poulson (1832) wrote in the introduction to his translation of Rafinesque’s monograph was, “… most of the Shells described in the following pages, have been placed in my cabinet by Mr. Rafinesque, with his labels and references.”

While this is perhaps good evidence that Rafinesque had identified the shells in the Poulson collection, it doesn’t make them a part of the original type series. In fact, it is conveniently overlooked that Rafinesque (1820) reported in his description of Obliquaria (Plagiola) depressa that, “J’ai déposé le seul individu que je posséde dans le muséum de M. J.D. Clifford à Lexington, avec toutes mes autres espèces...” (see footnote below) Are the Poulson specimens in Philadelphia the same ones that Rafinesque left in Lexington?

So, the specimen that we are illustrating this month is perhaps a Rafinesque ideotype (i.e., a specimen identified by the author but with no name-bearing status). But, before you get all cranked up that it should be designated as a neotype (ICZN, Art. 75), please keep in mind that 1) that would open a whole can of worms because the same situation exists for many of the Rafinesque "types" in the Poulson collection at the ANSP and 2) everyone has been satisfied by the status quo for more than a century.

It is convenient that Rafinesque’s name sounds like an adjective. This is a very Rafinesque state of taxonomic affairs.

Footnote: It is maybe slightly interesting to note that Poulson (1832) translated this as “with my other species” rather than “with ALL my other species.”


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Superfamily UNIONOIDEA Rafinesque, 1820
Family UNIONIDAE s.s.
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe LAMPSILINI Ihering, 2901

Genus Ptychobranchus Simpson, 1900

Species Ptychobranchus fasciolaris (Rafinesque, 1820)

To find out more about Ptychobranchus and other species described by Rafinesque, check out:
  • Johnson, R.I. & H.B. Baker. 1973. The types of Unionacea (Mollusca: Bivalvia) in the Academy of Natural Sciences of Philadelphia. Proceedings of the Academy of Natural Sciences of Philadelphia 125(9): 145-186.
  • Poulson, C.A. 1832. A Monograph of the Fluviatile Bivalve Shells of the River Ohio, containing twelve genera & sixty-eight species. J. Dobson, Philadelphia. 72 pp.
  • Rafinesque, C.S. 1820. Monographie des coquilles bivalves fluviatiles de la Riviere Ohio, contenant douze genres et soixante-huit especes. Annales Générales des Sciences Physiques 5 (13): 287-322, plates 80-82.
  • Vanatta, E.G. 1915. Rafinesque's types of Unio. Proceedings of the Academy of Natural Sciences of Philadelphia 67: 549-559.
  • Williams, J.D., A.E. Bogan & J.T. Garner. 2008. Freshwater Mussels of Alabama and the Mobile Basin in Georgia, Mississippi and Tennessee. University of Alabama Press, Tuscaloosa. 908 pp.

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