Leptammina Cedhagen, Gooday & Pawlowski, 2009, gen. nov.

Leptammina gen. nov.

Etymology: The first part of the name is derived from the Greek word leptos (λεπτος) meaning thin and refers to the thin test wall. The last part, -ammina, is a common ending of many generic names of agglutinating foraminiferans, particularly those with tests of sand grains, and is derived from the Greek word ammos (αΜΜος), meaning sand.

Type species: Leptammina flavofusca sp. nov.

Definition: Test free, monothalamous and more or less spherical, up to 2 mm diameter. Wall delicate, flexible, with inner organic layer overlain by fine, loosely agglutinated mineral grains. Single prominent circular aperture, which may protrude slightly from the test surface. Cell body with well-developed peduncular sheath.

Remarks: The two species described below look superficially different, particularly in terms of their colour. However, they are united by a number of test features, notably the wall structure, the presence of a peduncular sheath, and the size and form of the aperture. Moreover, our molecular analyses indicate that they are closely related (Fig. 6).

At least one globular, soft-walled monothalamous foraminiferan with a single aperture has been included in the genus Saccammina. Hedley (1962) placed his new species S. alba here, but with evident hesitation. He remarks — 'The present author has followed Le Calvez (1935) in placing a Foraminifer with a fragile, white shell in the genus Saccammina, the type species of which has a hard, brittle, ferruginous shell. If, in the future, sufficient becomes known about the various species of saccamminids to warrant a division into two genera, it seems likely that a basis for it may be found in the two shell types.' We agree that the differences between softwalled saccamminids in general and the type species Saccammina sphaerica Carpenter, 1869 are sufficient to justify the establishment of at least one new genus. The type species of the genus Saccammina is large, up to at least 4 mm diameter, with a rigid, fairly coarsely and firmly agglutinated wall. In these respects, it is unlike S. alba or the two delicate, soft-walled species described here. We establish the new genus Leptammina to accommodate our new species and S. alba. The last species, however, is included with some hesitation because of lack of molecular data.

Several other genera of soft-walled saccamminids have been described. Perhaps the most similar genus to Leptammina is Pilulinella (type species P. sphaerica Saidova, 1975). It is described by Loeblich and Tappan (1987) as being 'spherical', 1–2 mm in diameter with a single rounded terminal aperture and a wall composed of 'fine clay' particles on an organic base. The aperture in Pilulinella is reported to be a kind of double structure, although this could indicate the presence of a peduncular sheath. Unfortunately, not enough information is currently available about Pilulinella to determine how close it is to the new genus. However, the apparently greater size of the aperture may be one distinguishing feature. The diameter of the rim of the aperture is about one third of the test diameter in Pilulinella and about a fifth to a tenth of the test diameter in Leptammina.

Ovammina (type species O. opaca Dahlgren, 1962; synonym Dahlgrenia Lena, 1974) resembles Leptammina in having an agglutinated test wall made of minute mineral particles underlain by an organic layer. Both genera also have an entosolenian tube. It differs from Leptammina in being smaller (length130–700 µm) and ovoid, egg-shaped or fusiform rather than rounded in general test shape There is also no evidence in the new genus for the ring of accessory apertures around the main aperture that develop in Ovammina during gametogenesis.

Psammophaga (type species P. simplora Arnold, 1982) resembles Leptammina in having an outer finely agglutinating test layer and an inner organic wall. Both genera also have an entosolenian tube. One obvious difference is that Psammophaga ingests large quantities of mineral particles (Arnold 1982), a feature never observed in Leptammina. The test also has a relatively much thicker inner organic layer than that of Leptammina.

Like Saccammina, the genus Pilulina (type species P. jeffreysii Carpenter, 1875) is a heterogeneous taxon in need of revision. The species P. jeffreysii is large, up to at least 4 mm diameter, with a fairly rigid test wall and an elongate slit-like aperture. Pilulina does not have any similarity to Leptammina. However, some species assigned to this genus differ from the type species so much that their placement is questionable. Pilulina ovata Cushman, 1910, described from the North Pacific Ocean, has an oval test with a very thin, finely agglutinated wall. However, it is much larger than Leptammina and has an irregularly shaped aperture. Another species, Pilulina argentea Höglund, 1947, resembles superficially the "silver saccamminid" of Pawlowski et al. (2002, 2005) so much that we earlier believed them to be identical. Höglund (1947) included this species in the genus Pilulina with some hesitation. He wrote "Rather than erect a new genus, I am referring this species to Pilulina, although neither the aperture nor the structure of the wall exactly coincides with any of the hitherto described species belonging to this genus". Molecular evidence (see below) suggests that one undescribed species with a reflective, silver-coloured test wall is closely related to Leptammina.