Amusium precursor (Dall, 1898)
DALL, W. H. 1898.
Contributions to the Tertiary fauna of Florida. Silex Beds of Tampa and the
Pliocene Beds of the Caloosahatchie River. Part IV. I. Prionodesmacea: Nucula
to Julia. 2. Teleodesmacea: Teredo to Ervilia. Transactions of the Wagner Free
Institute of Science of Philadelphia, 3 (4): viii, 571-947 p., pls. 23-35. [p. 755]
1898 Pecten (Amusium) precursor Dall, 1898
Amusium (Amusium) precursor (Dall); H. I. Tucker-Rowland, 1938, The Atlantic and Gulf Coast
Tertiary Pectinidae of the United States, plate 2, figure 1.
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«Oligocene of the Chipola beds at Alum Bluff and on the Chipola River and elsewhere in these beds; Burns and Dall.
There are several species of Amusium ranging from the Oligocene to the recent fauna in this region. In general they appear extremely similar, so much so that such figures as are ordinarily given would show no differential characters. By careful and repeated study I find myself able to separate them by the umbonal sculpture, which differs in the different forms as follows: Nepionic shell perfectly smooth externally. 1. Shell more or less ovate: P. papyraceus Gabb. 2. Shell very large, orbicular: P. Mortoni Rav. Nepionic left valve with obsolete radii and often feeble concentric undulations: P. precursor Dall. Nepionic left valve with distinct flattened ribs with shallow channelled interspaces crossed by concentric, evenly spaced, not crowded, elevated lines : P. Lyoni Gabb. Left valve of the adult with obsolete rounded ribs extending, in the adult, well over the middle of the disk: P. ocalanus Dall. P. precursor is nearly as large as P. Mortoni, but slightly rougher and more convex when adult, the young are nearly orbicular; a distinct trace of Camptonectes striation, near the beak and submargins, may be discerned with a magnifier in a good light. Alt. 110, lat. 123, diam. 20 mm. The right valve is much flatter than the other. As the material is much broken up, it seemed hardly worth while to figure it.» WILLIAM HEALEY DALL, 1898
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«Amusium precursor (Dall, 1898) occurs in the Miocene Chipola Formation in Florida, a unit close in age to the Pungo River Formation (Gibson, 1967:643; Akers, 1972:9). Thus, A. precursor is of a generally comparable age to the specimens found in the Lee Creek Mine, but differs in having uniformly closely spaced internal lirae and finely impressed radial lines on the exterior.
As Dall (1898) did not select a holotype or illustrate any specimens of A. precursor, a lectotype is here selected and a lectoparatype illustrated (Plate 31: figures 5, 6). The lectotype is an articulated specimen, USNM 647532, from USGS 2213 in the Chipola Formation, one mile (1.6 km) below Bailey's Ferry, Chipola River, Florida. A number of problems are encountered in the selection of the lectotype. Among localities mentioned by Dall (1898:755) in his description of the species are Alum Bluff, from which no material could be found in the museum collections, and others along the Chipola River. Gardner (1926:50) and other writers have taken USGS 2212 (Chipola Formation at Ten Mile Creek) as the type locality, possibly because the label in the museum collections says "type locality" compared with "cotype locality" for USGS 2213; however, this entry of unknown origin has no standing. In the USNM catalogue for the lots from these localities, the number of specimens listed is fewer than are now in the type lots, indicating that specimens were added to the type lots, whether before or after Dall described this species is not known. The only specimen that can be identified unequivocally with Dall's original material is the largest specimen from USGS 2213, which agrees with measurements given in the original description. Therefore this specimen is picked as the lectotype. All the material in the lots from USGS 2212, 2213, and 2564 will be considered lectoparatypes as there is no feasible way to determine if, or which, specimens may have been added to the type lots, and all the material appears to be conspecific. A lectoparatype is illustrated (Plate 31: figures 5, 6) to show the interior lirae, as the lectotype is an indurated double valve, the interior of which cannot be observed.» GIBSON, T. G. 1987. Miocene and Pliocene Pectinidae (Bivalvia) from the Lee Creek Mine and adjacent areas. Smithsonian Contributions to Paleobiology, 61: 31-112, pls. 1-31. [p. 77, 78]
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Amusium precursor (Dall, 1898); T. G. Gibson, 1987, Miocene and Pliocene Pectinidae from the Lee Creek Mine and adjacent areas, plate 31, figures 5, 6.
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