 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations .(December 2014) |
| Sigmoilina Temporal range: Early Miocene - Recent. | |
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Scientific classification  | |
| Domain: | Eukaryota |
| Clade: | Sar |
| Clade: | Rhizaria |
| Phylum: | Retaria |
| Subphylum: | Foraminifera |
| Class: | Tubothalamea |
| Order: | Miliolida |
| Family: | Miliolidae |
| Subfamily: | Quinqueloculininae |
| 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).