Early Miocene; Extinct.
Original Description (from Dall, 1898, p. 648):
"Oligocene of the Chipola River, Florida, marl.
Shell rectangular, elongate, rather compressed, with low prosocoelous beaks, situated at about the anterior fourth of the whole length; right valve with about thirty-six flattened radial ribs, with much narrower interspaces; the anterior (twenty-two) ribs are mesially divided by a sharp groove and feebly rippled above; the posterior ribs are flat, smooth, and increase in width backward; the anterior end of the shell is evenly rounded, the base straight and parallel with the hinge-line, the posterior end wider, a little produced below and with a conspicuous angle above; cardinal area long, very narrow, with one or two grooves, and bordered behind with an elevated margin; hinge-line straight, long, with numerous small, uninterrupted teeth very short mesially, longer and somewhat more oblique distally; inner margin of the valves fluted, shell thin and delicate. Lon. 20, alt. 10.5, semi-diam. 4.5 mm.
Only two right valves of this little species have been examined. It resembles the young of A. hypomela but is immediately distinguishable by its more compressed and rectangular form and smooth, flat posterior ribs."
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