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| Sigmoilina Temporal range: Early Miocene - Recent. | |
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| Genus: | Sigmoilina Schlumberger 1887 |
Sigmoilina is a miliolid genus, referring to the foraminiferal order Miliolida, characterized by an assymmetricall biconvex test formed by strongly overlapping chambers, one-half coil in length, that form a sigmoid (S-shaped) curve in cross section. The strongly overlapping chambers obliterate earlier ones from view resulting in the compressed biloculine appearance, differing from the squat, depressed biloculine form of Pyrgo and Biloculina . The test, as for all Miliolida, is porcelaneous and imperphorate, the terminal aperture, with tooth, the only point of egress and ingress for the animal.
Sigmoilina is a protist, or proctoctist according to B.K. Sen Gupta, 1999, included in the miliolid family Hauerinidae (Loeblich & Tappan, 1988). It was previously assigned to the Miliolidae (Loeblich & Tappan, 1964) when foraminifera were regarded as comprising a suborder (Foraminiferida).