Belongs within: Rotaliida.
The Chilostomellidae are a group of Foraminifera known from the Jurassic to the present day. In the genus Chilostomella the test is planispiral and involute with a narrow interiormarginal equatorial slit for the aperture (Loeblich & Tappan 1964).
Chilostomellidae: deep forams
Published 28 December 2019
The specimen in the figure above is a fairly typical representative of the Chilostomellidae, a cosmopolitan family of forams known from the Jurassic to the present day. Members of this family have a translucent calcareous test with chambers arranged in a trochospiral (broad conical) or planispiral (flat spiral) pattern. The chambers of each spiral are expanded to cover over the prior spirals so only the outermost spiral is generally visible. The aperture of the test in the final chamber is a narrow slit along the margin with the underlying chamber (Loeblich & Tappan 1964).
Despite their long history and wide distribution, I get the general impression that chilostomellids are not usually abundant. They are generally restricted to deeper waters, more than 100 m below the surface (Cushman et al. 1954). Members of the genus Chilostomella, at least, have commonly been regarded as associated with low-oxygen environments. However, it has also been suggested that their favoured conditions are not so much a question of low oxygen as high organic flux (Jorissen 2002). Perhaps the best location to find chilostomellids would be around sites where dead animals and seaweeds have fallen to the deeper waters below.
Systematics of Chilostomellidae
Characters (Loeblich & Tappan 1964, as Chilostomellinae): Test trochospiral, with few chambers to whorl, planispiral and involute; aperture interiomarginal on umbilical side.
<==Chilostomellidae [Chilostomellinae]
|--Bagginoides Podobina 1975HW93
|--Allomorphina Reuss in Cžjžek 1849 [Allomorphininae]LT64
| |--*A. trigona Reuss 1850LT64
| |--A. cretaceaM08
| |--A. pacificaJW99
| `--A. paleocenicaQS03
|--Allomorphinella Cushman 1927LT64
| `--*A. contraria (Reuss 1851) [=Allomorphina contraria]LT64
|--Chilostomelloides Cushman 1926LT64
| `--*C. oviformis (Sherborn & Chapman 1886) [=Lagena (Obliquina) oviformis]LT64
`--Chilostomella Reuss in Cžjžek 1849LT64
|--*C. ovoidea Reuss 1850LT64
`--C. oolina Schwager 1878H03
*Type species of generic name indicated
References
Cushman, J. A., R. Todd & R. J. Post. 1954. Recent Foraminifera of the Marshall Islands. Bikini and nearby atolls, part 2, oceanography (biologic). Geological Survey Professional Paper 260-H: 319–384, pls 82–93.
[H03] Hanagata, S. 2003. Miocene-Pliocene Foraminifera from the Niigata oil-fields region, northeastern Japan. Micropaleontology 49 (4): 293–340.
[HW93] Hart, M. B., & C. L. Williams. 1993. Protozoa. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 43–70. Chapman & Hall: London.
[JW99] Jian, Z.-M., L.-J. Wang, M. Kienast, M. Sarnthein, W. Kuhnt, H.-L. Lin & P.-X. Wang. 1999. Benthic foraminiferal paleoceanography of the South China Sea over the last 40,000 years. Marine Geology 156: 159–186.
Jorissen, F. J. 2002. Benthic foraminiferal microhabitats below the sediment-water interface. In: Sen Gupta, B. K. (ed.) Modern Foraminifera pp. 161–179. Kluwer Academic Publishers: Dordrecht.
[LT64] Loeblich, A. R., Jr & H. Tappan. 1964. Sarcodina: chiefly “thecamoebians” and Foraminiferida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt C. Protista 2 vol. 2. The Geological Society of America and The University of Kansas Press.
[M08] McMillan, I. K. 2008. Reappraisal of foraminiferal assemblages of the Santonian–Campanian Mzamba Formation type section, and their correlation with the stratigraphic succession of the KwaZulu Basin. African Natural History 4: 25–34.
[QS03] Quattrocchio, M. E., & W. A. S. Sarjeant. 2003. Dinoflagellates from the Chorrillo Chico Formation (Paleocene) of southern Chile. Ameghiniana 40 (2): 129–153.