Amusium alazanum Cooke, 1928
COOKE, C. W. 1928. New Vicksburg (Oligocene) Mollusks From Mexico. Proceedings of the United States National Museum, 73 (10): 1-11, pls. 1, 2. [p. 10, pl. 2, figs. 7, 7a]
1928 Amussium [sic] alazanum Cooke, 1928
C. W. Cooke, 1928, plate 2.
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«Shell small, thin, suborbicular, nearly equilateral, somewhat tumid medially but compressed laterally, surface smooth except for very faint growth lines and Camptonectes striations; apical angle about 115°; ears large, subequal; internal sculpture of ten strong, round, threadlike ribs extending from the umbo, where they are covered by a thin layer of shell, nearly to the margin and ending in flattened nodes. Height, 7.8 mm.; width, 7.6 mm.
The length and number of internal ribs are variable. Another specimen nearly the same size as the type has 11 ribs, and the ribs extend only about two-thirds the distance to the margin. The distal third is smooth and very thin. Occurrence.— Oligocene, Alazan clay, Rio Buena Vista west of Alazan (M. 49 V.) (type) and at La Ceiba Crossing, 9.8 kms. above Tumbadero (M. 52 V., M. 54 V.). Type.— Cat. No. 352710, U.S.N.M.» CHARLES WYTHE COOKE, 1928
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«Named for the Alazan Formation of La Ceiba, Buena Vista River, Veracruz State, Mexico, which contains the typical fauna, this province is still poorly known and unestudied. The type formation, and only extensive collecting site, contains a relatively impoverished fauna but enough evidence is present to demonstrate that this area was distinct from the subprovinces to the north and south (Cooke, 1928). Although sharing several gastropods with the Vicksburgian and Hernandoan Subprovinces, such as the previously mentioned Torcula mississippiensis, Distortio crassidens, Phalium caelatura, and Sinum mississippiensis, and the turrid Pleurofusia servata and the natacid Polinices byramensis (Natacidae), the Alazanian Subprovince contained several unusual and distinct endemic gastropod abd bivalve species such as Ancilla alazana (Olividae), Amusium alazanum (Pectinidae), the bizarre Thatcheria-like turrid "Pseudotoma" alazana (representing an undescribed endemic genus), and species radiations of the turrid genera Scobinella, Gemmula, Glyptotoma, and Paraborsonia. Of special interest was the presence of the endemic melanelid gastropod genus Protonema (P. bartschi).»
PETUCH, E. J. 2003. Cenozoic Seas. The view from Eastern North America. CRC Press, 308 p., pls. 1-98, figs. 1-34. Boca Raton, Florida. [p. 26, 27]
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