Harry,

Henk's comments are to the point.

As the southernmost Cretaceous strata along U.S. Highway 231 are at Troy,
more than 50 miles north of the Alabama-Florida state line, the locality
cannot be "just above the Florida line in S. E. Alabama" if this is Exogyra.
The genus became extinct at the end of the Cretaceous.

Exogyra belongs to the "other" oyster family, the Gryphaeidae, and is
related to Jurassic Gryphaea, Cretaceous Pycnodonte, and modern Hyotissa
(which seems to have descended from Pycnodonte). They were very common in
the Mesozoic, and large Hyotissa is common in the Pliocene beds at Sarasota,
Florida. Like modern oysters, the gryphaeids are incredibly variable in form
from individual to individual, and one has to look for a few distinctive
characters. They have one large adductor scar and hinges much like ostreids.
The adults usually have both laminated and finely vesicular ("bubbly") shell
structure (invisible unless the shell is broken; and may be filled in with
later calcite deposits) and a shelf-like structure along the pallial line.
There may be an odd pattern of little ridges (chomata) on either side of the
hinge, though it is not always present.

Exogyra is very inequivalve. The larger valve is boldly curved in a low
spiral, and tends to be thick-shelled. This is the valve that starts out
being attached, commonly to another mollusk shell (frequently to a pteriid
or another Exogyra), but most individuals attach to such small bits of shell
that they evidently don't need this support as adults. The smaller valve is
almost flat in most species and may even be concave, as in some modern
scallops. The ornament is superficial and weathering commonly removes it,
leaving a smooth valve and complicating identification of species.
Collectors who don't know Exogyra well tend to pick up the larger valves and
leave the less attractive smaller valves behind, but the smaller valves are
actually easier to identify to species.

In life, Exogyra probably rested on soft substrates (mud and sand) with the
larger valve embedded in the substrate, as in some modern scallops. The
valves commonly are extensively bored by sponges or other organisms, and may
be encrusted with bryozoans and so on.

With their thick, calcitic shells strewing some outcrops in abundance, the
gryphaeids are well known to geologists, who use them to date the ages of
Cretaceous strata. There is an extensive literature on them. For the
southeastern U.S., I recommend:

Stenzel, H. B., 1971, Oysters. In Moore, R. C., ed., Treatise on
invertebrate paleontology, part N (Bivalvia), v. 3, p. N953-N1224.
Geological Society of America and University of Kansas Press, Boulder,
Colorado, and Lawrence, Kansas. [Including brief descriptions and photos of
all oyster genera whether living or fossil, plus an extensive introduction
on their ecology, growth, etc. In print; available from the Geological
Society of America.]

Stephenson, L. W., 1914, Cretaceous deposits in the eastern Gulf region and
species of Exogyra from the eastern Gulf region and the Carolinas: U.S.
Geological Survey, Professional Paper 81, 77 p., 21 pl. [The stratigraphy
may be out of date, but the descriptions and full-size photos of species of
Exogyra have never been surpassed, and their taxonomy has been only slightly
modified. Out of print, but not a rare work; many university libraries have
it, often in the documents section.]

Many of the Cretaceous oysters, both ostreids and gryphaeids, had more
regular forms than their modern counterparts, and some are quite attractive.
Take a look at Charles Newsom's website, as Henk recommended.

Andrew K. Rindsberg
Geological Survey of Alabama