Dr. Rob Dillon, Coordinator

Thursday, April 6, 2023

Growing up with periwinkles

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  Growing Up With Periwinkles.  Pp 89 – 99 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other Essays.  FWGNA Project, Charleston, SC.

Back in 2014 I posted a reminiscence on this blog [1] about growing up a frustrated young malacologist in the little city of Waynesboro, Virginia.  From my house it was just a couple hundred yards through old fields to the South River, a tributary of the Shenandoah.  And I wrote:

“The rocks were (and still are) covered with little black snails we called ‘periwinkles.’ I didn't collect them for the same reason I didn't collect ants.  They were dirt-common, and (in retrospect) I think my collecting button only got pushed when I saw something that seemed unusual, which periwinkles were not.  And I knew for a fact that, just as was the case with ants, I could not identify those periwinkles, even if I dug through every "Mollusca" card in the University of Virginia Library catalog, which I had.”

I dropped the subject of “periwinkles” at that point of the essay, in favor of another even more common and even more frustrating freshwater snail, that of dirt-brown hue, Physa.  So, this month we’ll pick at that little-black periwinkle thread again and see what unravels behind it.

Because those periwinkles were the most conspicuous gastropod of my youth.  Further south up the Great Valley of Virginia my father and I spent many lovely hours fishing in the James River and tributaries such as the Cowpasture and the Jackson.  And the next river heading south was the Roanoke, where I had dozens of cousins, and the next river was the New River, in whose bosom I wiled away four sweet, gauzy years of college.  And in all of those river systems, in every rapid or riffle big enough for a sun perch or a redeye bass, the rocks were covered with little black periwinkles.

Jackson R, Va.

It was only after my arrival at Virginia Tech in the fall of 1973 that I was finally able to identify “periwinkles,” poking through the malacological journals, reading backward through the literature, discovering the work of Calvin Goodrich [2]Nitocris carinata!  Authored by the French zoologist Jean Guillaume Bruguière in 1792, “Bulimus” carinatus was the second freshwater gastropod [3] described from the continent of North America, “les eaux douces de la Virginie[4].  Goodrich gave its range as “New York to North Carolina.”

But almost immediately after I had solved the mystery of the specific identity of the periwinkle, a fresh mystery presented itself.  To what genus should the periwinkle be assigned?  Although Goodrich advocated Nitocris (H. & A. Adams 1854), Dr. E. F. (Fred) Benfield, my undergraduate advisor, preferred old Joe Morrison’s [5] Mudalia (Haldeman 1840).  Tryon [6] and Walker [7] preferred Anculosa (Say 1821).  And Juan Parodiz, who had authored the best review of the systematics and distribution of periwinkles to that date [8], made a case for Leptoxis (Rafinesque 1819), pushing Mudalia underneath Leptoxis as a subgenus.  It turned out that I had stumbled into one of the most contentious issues in American malacology.

There were actually two layers of controversy. The first was whether Rafinesque had described his (1819) Leptoxis with sufficient clarity that it might take precedence over Say’s (1821) Anculosa.  And the second was whether either of two subsequent names, Mudalia of Haldeman or Nitocris of H & A Adams, both originally proposed as subgenera, might deserve elevation to the full genus level.  Iced over those layers of controversy was the situation with a larger-bodied group that Tryon, Walker and Goodrich all referred to as Eurycaelon (Lea, 1864), but which Joe Morrison insisted was in error, for which he proposed the new name Athearnia in 1971, more below.  And then there was a funny little sprinkle of walnuts on top about Alleghenya.

Who among my vast and diverse readership has ever heard of Dr. Kenneth J. Boss?  I didn’t think so.  Boss was a student of Bill Clench, and heir to the curatorship of malacology at the MCZ Harvard.  He had a quiet career, bless his heart.  But in 1967 he and his mentor published a little paper ignoring Leptoxis, discounting Mudalia and Nitocris, and proposing to classify periwinkles in a new subgenus of Anculosa they called “Alleghenya[9].

L. carinata from Parodiz [8]
I met Ken Boss in January of 1977, when I was interviewing prospective graduate advisors.  And I asked him what the hell he was thinking with this Alleghenya thing, which was a pretty cheeky way for a 21-year-old student to address a Harvard professor.  And he admitted his mistake and apologized [10].

I ultimately chose Mudalia as the genus under which to file the periwinkles I collected during the course of my undergraduate thesis research [11], because that’s the name my faculty advisor preferred.  But three years after I graduated, Jack Burch [12] brought the tablets down the mountain, and engraved upon them was Leptoxis at the genus level, with Mudalia a subgenus beneath it.  Burch didn’t offer any rationale, but neither did God [13].  And Leptoxis it has been, ever since.

But here’s a fresh puzzle, now that we’ve identified those periwinkles that covered the rocks in the South River of my youth as Leptoxis (aka Nitocris, aka Mudalia) carinata (Brug 1792).  When I left home for college, I drove south up the Great Valley through the James River drainage, which was chock full of L. carinata, and the Roanoke River drainage, which was chock full of L. carinata, and then up the long grade to the New River Plateau.  But for reasons unknown and unexplained, Calvin Goodrich [14] identified the periwinkles of the New River drainage as Nitocris (aka Leptoxis, aka Mudalia) dilatatus (Conrad 1834), not carinata.

True, the vast herds of periwinkles covering the rocks in the New River tributaries outside my dorm room window in Blacksburg bore shells lacking that carination so often prominent on the shells borne by the herds of periwinkles outside my bedroom window in Waynesboro.  But by the time I was 20 years old I had been kicking those little things off rocks for 15 years, all over the Commonwealth of Virginia, and I knew how variable that little black shell could be.

And true, the New River drains west to the Ohio, while the Shenandoah, James, and Roanoke drain east to the Atlantic.  But even in my tender youth I was already tremendously impressed by the extensive geographic ranges demonstrated by pleurocerid snails.  I didn’t understand it.  Still don’t [15].  But as Goodrich observed, Leptoxis carinata is found in every Atlantic drainage from the Susquehanna way up in New York to the Broad/Congaree way down in North Carolina, and on that scale the 10 mile climb up I-81 leaving the Roanoke River Valley and entering the New River plateau doesn't amount to a hill of beans.  And ditto continuing back down the Blue Ridge on I-77 south from the New to the Yadkin/Pee Dee.  The little black periwinkles swap carinata-dilatata-carinata over that 50 mile stretch, really?

"Mudalia dilatata" from my ugrad thesis [11]
Moreover, by my undergraduate years I was already becoming obsessed with another local pleurocerid, Pleurocera (aka Goniobasis, aka Elimia) proxima, the range of which did extend across the Dan/Roanoke through the New River into the Yadkin/Pee Dee, with no apparent interruption.  Why should Goodrich distinguish the New River Leptoxis with a unique name, “Nitocris dilatata,” while identifying all the Pleurocera across the entire Roanoke-New-Pee Dee region as “Goniobasis proxima?"

It took thirty years, but John Robinson and I ultimately answered that question in 2009, with the survey of CO1 sequence divergence we called “The Snails the Dinosaurs Saw [16].”  Although the focus of that study was mitochondrial superheterogeneity within populations, we did not neglect interpopulation divergence as well.  Our pair of carinata/dilatata populations from the Yadkin/New were more similar to each other than to their nominal conspecifics in a carinata/dilatata pair sampled from the James/Greenbrier 150 km north.  Conrad’s (1835) dilatata is a junior synonym of Bruguiere’s (1792) carinata.

But back to the thread of my story.  It was also during my undergraduate years at Virginia Tech that I was first introduced to the second-oldest species of Leptoxis in North America, Thomas Say’s “Melania” praerosa, described from the Falls of the Ohio in 1821.  The introduction came in 1975, during the summer after my sophomore year, when I was blessed to be offered a temporary job with the Tennessee Valley Authority in Norris, and it was my new co-worker Steve Ahlstedt who did the honors.  Interestingly, Steve called those little snails “Anculosa subglobosa,” which was my first introduction to a specific controversy almost as idiosyncratic as the controversy over the genus.

Thomas Say described “Melania subglobosa” from the North Fork of the Holston River in 1825, and the species was considered distinct and valid by all authors through the 19th and most of the 20th century, including by Tryon [6] and Goodrich [14], both of whom assigned it to Anculosa. But in 1980 Jack Burch [12] synonymized subglobosa under Say’s praerosa without explanation [17] or even comment.  He simply wrote, in the caption of the figure printed on his page 157: “FIG. 480. L. subglobosa = L. praerosa.” And just like that, Anculosa subglobosa (Say 1825) was gone.

By whatever name, I found Leptoxis praerosa populations as widespread and dense in the rich headwaters of the Tennessee River to the west of the New River as L. carinata were in the Yadkin/PeeDee to the South and the Dan/Roanoke to the east.  Interestingly, carinata has apparently dispersed into the upper Holston/Tennessee with praerosa, and praerosa seems to have spread into Walker Creek of the New River with carinata, and in neither of these drainage systems is any hybridization in evidence, to my eye, in any case.

Leptoxis praerosa, thanks to Chris Lukhaup
The two species are easy to distinguish by the relative sizes of their body whorls, L. praerosa’s being so large as almost to obscure the apex entirely, smoothly rounded and entirely unsculptured, with no hint of carination.  They look like pebbles, with a bit more mobility, but less personality.  It is perhaps their unmitigated plainness that has spared them the metastasis of synonymy that has afflicted almost every other biological species of pleurocerid snail in North America.  All across their eight-state range, Goodrich listed only nine synonyms for praerosa and four for subglobosa, all of which are too obscure to mention.

Because populations are so widespread throughout the Ohio, Cumberland, and Tennessee drainages, and because (up until recently) their identity was uncontroversial, Leptoxis praerosa was an easy choice as a control for two allozyme studies I published in the 1990s, one of Leptoxis crassa [18] and a second of the Alabama taxa L. picta, L. ampla, L. plicata, and L. taeniata [19].  In that former study, Steve Ahlstedt and I confirmed reproductive isolation at four loci between sympatric populations of L. praerosa and L. crassa co-occurring in the Sequatchie River, about 10 miles west of Chattanooga.

Steve and I identified the population of big-bodied pleurocerids we sampled from the Sequatchie as “Athearnia anthonyi.”  That was the name Steve wrote on the label in the bag of snails he shipped to me on dry ice, which I transferred to my data sheets, anyway.  In 1971 old Joe Morrison proposed Athearnia as a genus to hold two nominal species of large-bodied pleurocerids that Tryon, Walker and Goodrich had all previously assigned to Eurycaelon (Lea 1864): Melania crassa Haldeman 1842 and Melania anthonyi Redfield 1854 [20].  Burch demoted Athearnia to subgeneric rank under Leptoxis in 1980, and lowered anthonyi to subspecific level under crassa.  But in 1994 the US Fish & Wildlife listed “Athearnia anthonyi” as an endangered species, declaring A. crassa extinct in the process [21], pretty much freezing the 1971 science forever [22].

Leptoxis crassa in Limestone Ck.

So shortly after Steve Ahlstedt and I published our paper on L. crassa, Chuck Lydeard sent me a big batch of Leptoxis from the Mobile Basin [19].  I opened the cooler to find eight populations of four nominal species: three of L. ampla from shoals of the Cahaba River, two of L. taeniata from tributaries of the Coosa, two of L. plicata from Locust Fork (of the Black Warrior) and one of L. picta from the main Alabama River.  The levels of allozyme divergence among six of those eight populations turned out to be comparable to that demonstrated by a set of three L. praerosa control populations I collected from Tennessee drainages separated by similar distances.  Our analysis suggested that ampla, taeniata, and picta were all conspecific, Leptoxis picta (Conrad 1834) being the oldest name for the lot.  The average allozyme divergence demonstrated by the L. plicata populations of the Black Warrior system was consistent with their status as a distinct biological species.

So if you had asked me ten years ago, I would have listed six valid species of Leptoxis in North America: carinata, praerosa, crassa, picta, plicata, and maybe that weirdo way out in the Ozarks, Leptoxis arkansensis (Hinkley 1915), I have no reason to doubt.  That six-species hypothesis comes from over 60 years of field experience with Leptoxis in the creeks, thousands of hours of laboratory research on the genetics of pleurocerid populations, and an intimate familiarity with 200 years of accumulated scientific literature.  It is science.  It is a testable model of the natural world.  Next month, we test it.


[1] This was the first episode in what turned out to be a five-part series on the evolution of our understanding of the North American Physidae.  If you’re curious about the entire Physa story, the best approach might be to go to the final installment [6Dec18] and read backwards.  Otherwise:

  • To Identify a Physa, 1971 [8Apr14]

[2] Goodrich, C. (1942) The Pleuroceridae of the Atlantic coastal plain.  Occasional Papers of the Museum of Zoology, University of Michigan 456: 1 – 6.  For an appreciation, see:

  • The Legacy of Calvin Goodrich [23Jan07]

[3] To the German naturalist Johann Friedrich Gmelin goes the honor of describing the first freshwater gastropod endemic to North America, “Buccinum” (Pleurocera) virginicum in 1791.

[4] Bruguière, Jean Guillaume (1792) Vermes ('worms') in Daubenton's Encyclopédie Méthodique.

[5]  Morrison, J. P. E. (1954) The relationships of Old and New World Melanians. Proc. U. S. Nat. Mus. 103: 357- 394.  For context, see:

  • Joe Morrison and the great Pleurocera controversy [10Nov10]

[6] Tryon, G. W. (1873)  Land and Freshwater shells of North America Part IV, Strepomatidae.  Smithsonian Miscellaneous Collections 253: 1 - 435.  For a thumbnail biography, see down below in my Isaac Lea bio:

  • Isaac Lea Drives Me Nuts [5Nov19]

[7] Walker, B. (1918)  A synopsis of the classification of the freshwater Mollusca of North America, North of Mexico, and a catalogue of the more recently described species, with notes.  Univ. Mich. Mus. Zool. Misc. Publ. 6: 1 - 213.  For an appreciation, see:

  • Bryant Walker’s sense of fairness [9Nov12]

[8] Parodiz, J. J. 1956.  Notes on the freshwater snail Leptoxis (Mudalia) carinata.  Annals of the Carnegie Museum 33: 391 - 405.

[9] Clench, W.J. and K. J. Boss (1967) Freshwater Mollusca from James River, Va., and a new name for Mudalia of authors.  Nautilus 80: 99 – 102.

[10] But Ken Boss was never going to win me as a graduate student, in any case.  George Davis had greatly impressed me at the AMU meeting in the summer of 1976, and was a very gracious host in Philadelphia, and I was hooked.  Burch had the early lead, but by the winter of 1976-77 had blown it

[11] Dillon, R. T., Jr. (1977) Factors in the distributional ecology of upper New River mollusks (Va/NC).  Undergraduate Research Thesis, Virginia Tech, Blacksburg. 59 pp.  [pdf]

[12] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[13] Actually, He did, for numbers two and four only.  But for the rationale of number two, at least, it is difficult to be thankful.

[14] Goodrich, C. 1940. The Pleuroceridae of the Ohio River drainage system.  Occas. Pprs. Mus. Zool. Univ. Mich., 417: 1-21.

[15]  Well, the older I get, the more convinced I become that extensive pleurocerid ranges such as those displayed by Leptoxis carinata and Pleurocera proxima are the product of great age and extremely unlikely aerial dispersal events.  See the “jetlagged wildebison model” here:

  • Mitochondrial superheterogeneity: What it means [6Apr16]
  • Accelerating the snail’s pace, 2012 [24Apr17]

[16] Dillon, R. T., Jr. and J. D. Robinson (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11.  (Rosemary Mackay Award)  [pdf]  For more, see:

  • The snails the dinosaurs saw [16Mar09]

[17] The ultimate cause may have been sloppiness.  There is a line space missing between Goodrich’s treatment of Anculosa praerosa and his treatment of Anculosa subglobosa at the bottom of page 20 in his 1940 paper, which pulls subglobosa up underneath Goodrich’s list of nine praerosa synonyms.  Is it possible that Burch thought Goodrich was listing subglobosa as a tenth?

[18] Dillon, R. T., and S. A. Ahlstedt (1997) Verification of the specific status of the endangered Anthony's River Snail, Athearnia anthonyi, using allozyme electrophoresis. The Nautilus 110: 97 - 101. [pdf]

[19] Dillon, R.T., and C. Lydeard (1998) Divergence among Mobile Basin populations of the pleurocerid snail genus, Leptoxis, estimated by allozyme electrophoresis.  Malacologia. 39: 111-119. [pdf]  For more, see:

  • Mobile Basin II: Leptoxis lessons [15Sept09]
  • Intrapopulation gene flow, the Leptoxis of the Cahaba, and the striking of matches [2Nov21]

[20] Morrison, J. (1971) Athearnia a new name for a genus of pleurocerid snails.  The Nautilus 84:110 – 111.

[21] U.S. Fish and Wildlife Service (1994) Endangered and threatened wildlife and Plants; Determination of endangered status for the Royal Snail and Anthony’s Riversnail.  Federal Register 59: 17994 – 17998.  [FR-1994-04-15]

[22] This sort of thing pisses me off royally.  Science and public policy are not compatible.  And anybody who thinks otherwise is sharpening a knife to kill the former.


  1. Not interested in waiting a month? Curious to see what modern science has to say? Spoilers ahead: https://ssbbulletin.org/index.php/bssb/article/view/8419/7556

    1. You are quite the seer, my anonymous friend! The research of our colleague Nathan Whelan will indeed play a central role in next month's episode. Trust you'll tune in, nonetheless!