Early Miocene; Extinct.
Original Description (from Gardner, 1947, p. 557):
"Shell rather large and very fragile. Spire high scalariform. Apex decorticated in all available material. Whorls probably 7, increasing uniformly in size, all but the earliest strongly tabulated; sides of whorls nearly vertical; shoulder angle acutely rounded. Body only feebly inflated, smoothly rounded at the base. Surface smooth except for faint incremental striae and vaguely suggested spirals. Suture line distinct. Aperture wide, the exact outline lost by the imperfect labrum. Outer lip thin and sharp, patulous anteriorly, its margin broken away. Inner lip sigmoidal, reverted and reinforced anteriorly by a rather heavy callus. Parietal wall thinly washed. Umbilicus imperforate, evenly glazed.
Dimensions of holotype: Height, 37.5. millimeters; maximum diameter (outer lip lost), 25± millimeters.
Holotype: U. S. Nat. Mus. No. 136073.
Type locality: No. 2646, Oak Grove, Yellow River, Okaloosa County, Fla.
Pachycrommium dodonum is separated from Amauropsis floridana Dall of the Ballast Point silex beds by the larger size, more slender, less inflated outline and relatively higher spire, more strongly shouldered whorls, and more patulous anterior extremity. Amauropsis burnsii Dall is a smaller, more acutely tapering shell.
Some of the examples of Amauropsis Guppyi gurabensis Maury approach dodonum very closely, but the number of whorls seems a little higher and the shoulder flattening less pronounced in the West Indian forms. The individual variation, as Miss Maury noted, seems unusually high."
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