The large shells of the genus Ecphora constitute one of the most unusual and diagnostic types of fossil in the Maryland Miocene fauna. One of these shells (Pl. 25, fig. 1) furnished the material for the one of the first illustrations of a fossil from North America by Martin Lister in 1770, and these shells are still the most sought for prizes by the collectors of Maryland fossils. The form figured in 1770 (Echphora quadricostata) appears to be that now known as Ecphora gardnerae gardnerae Wilson (Pl. 25, fig. 2), a species that is distinguished by the four strong "T"-shape ribs. It is the common and characteristic species of the St. Marys fauna, but occasional specimens in the Choptank fauna cannot be distinguished from it. More typical of the Choptank fauna is Ecphora meganae (Ward and Gilinsky) (Pl. 25, fig. 3), in which the four ribs lack the "T"-shaped outer expansion of the typical St. Marys form and the umbilicus is rather widely flaring. This variety is present, though not common, in the Calvert Formation where the genus is represented by E. tricostata Martin (Pl. 25, fig. 4) with three strong ribs and a weaker fourth one at the base of the series.
These three species appear to represent an evolutionary sequence in which the three strong ribs of E. tricostata, which is common in the Calvert and rare in the lower part of the Choptank Formation, are retained and a fourth strong rib added by the increase in strength of the basal rib to form the typical Ecphora meganae, rare in the Calvert and common in the Choptank fauna. The ribs assume a "T"-shaped form in the St. Marys fauna. The specimens found to the south in Virginia and North Carolina lack this type of ribbing even though they are otherwise typical of E. gardnerae gardnerae and come from strata that are contemporaneous with those of the St. Marys Formation.
A noteworthy feature of the specimens of the genus Ecphora is that the shells retain a brown to reddish brown color even when all other fossils in the fauna have assumed a chalky white tint.
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