I believe the determination of two separate species still stands; in
his Compendium of Bivalves Markus Huber presents  them as separate
species (page 118) with the ranges essentially as noted in this
posting

On 7/30/15, Harry Lee <[log in to unmask]> wrote:
> I don't recall hearing anything substantive on the matter for about
> six years:
> <https://listserv.uga.edu/cgi-bin/wa?A2=ind0910B&L=CONCH-L&P=R413&X=A36E288A086962535B&Y=shells%40hglee.com>.
>
> Harry
>
>
> At 11:04 AM 7/30/2015, Derrick Hudson wrote:
>>Hello,
>>
>>     I know this is a very old string, but I'm new to CONCH-L and am
>> catching up. The paper
>>Genetics and taxonomy of ribbed mussels (Geukensia spp.), By Sarver
>>et al. 1992 addresses this question fully. As for my experience I
>>believe that G.demissa is the East coast species that spans from
>>Rhode Isaland [sic] to South Florida and that G.granosissma [sic] is
>>the Gulf of Mexico species. I personally have not found any
>>G.demissa North of the Keys.
>>
>>Derrick
>>
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