Hi Harry,

No problem. The "Treatise on Invertebrate Paleontology" is published by the
Geological Society of America:

http://www.geosociety.org/

Exogyra costata was named in this article (v. 2, p. 43; 1820):

Say, Thomas, 1819-20, Observations on some species of zoophytes, shells, &c.
principally fossil: American Journal of Science & Arts, series 1, v. 1, no.
4, p. 381-387 (1819); v. 2, no. 1, p. 34-45 (1820).

--which I think was reprinted 76 years later in:

Say, Thomas, 1896, A reprint of the paleontological writings of Thomas Say;
with an introduction by G. D. Harris: Bulletins of American Paleontology, v.
1, no. 5, p. 271-354.

--a publication that might be slightly more accessible.

Andrew K. Rindsberg
Geological Survey of Alabama