I have already written about the Polygyra species that Thomas Say described in 1818, which he had collected a year earlier during an expedition to Florida. Another Polygyra species from Florida, P. cereolus, was described by Muhlfeld (as Helix cereolus) in 1816 or 18181. This species is difficult to distinguish from Say's P. septemvolva. What I have identified here as P. cereolus may very well be P. septemvolva. In any case, the exact identification doesn't really matter for my purposes, because what I am going to say here applies to both species.
In my previous post I mentioned that Say gave separate dimensions for the shells of what he claimed were female and male Polygyra. Being pulmonate snails, they are, of course, hermaphrodites. The dissection below shows the lower genitalia of a specimen of P. cereolus.
The dimensions and shapes of the shells of P. cereolus and P. septemvolva are quite variable. The flatter shells of either species approach to being planispiral. The picture below shows the cross section of a P. cereolus shell.
1. See the footnote 2 on p. 582 of Pilsbry's Land Mollusca of North America, 1:2, 1940.