Frenopyxis bipilata (Baković, Siemensma, Baković and Rubinić, 2019) comb. n.
Diagnosis: In dorsal view, the test is rounded and usually slightly wider than long; in lateral view, the height of the test is more or less half (0.4-0.6) of the diameter of the test. The dorsal part gradually lowers towards the anterior side, the ventral surface is flattened. The test is built of granular organic cement, yellowish to brown, more or less embedded with small mineral particles. Some tests lack these xenosomes, others are more densely covered. The test never bears spines. The aperture is eccentric, invaginated and ovoid, elliptical or circular. The distance from the center of the aperture to the anterior edge is approximately 0.4 (0.37-0.46) of the test length. The edge of the pseudostome curves inward, creating a small but clear ring that is usually slightly darker than the rest of the test. The anterior part of the edge is higher and always connected to the inner anterior-dorsal wall by two struts. Between those struts there is a more or less U-shaped opening, the size of which is variable per specimen, usually large, but sometimes smaller (Fig. 5). Lobopodia as common for this genus. Nucleus not observed.
Dimensions: Length of the test 52-81 μm (± 61.7 μm); width 55-75 μm (± 63.0 μm); height 28-43 μm; diameter of the pseudostome 12-22 X 11-25 μm (± 17.2 X 17.8 μm) (n=29).
Ecology: Frenopyxis bipilata inhabits sediments in aquatic and terrestrial habitats and transitional habitats in freshwater karst caves in Croatia and Bosnia and Herzegovina. It is common in habitat whose energy input is dependent on the seeping water from the surface, on exogenous flooding and on the habitats that where additionally enriched with bats guano. All habitats, where this species was found, are out of the reach of the sunlight during the whole year. The recorded temperature on habitats of this species is ranging from 18°C (e.g. Topla peć) to the 4,1°C (e.g. Veternica cave).
Biogeography: This species was detected in seventeen caves from Croatia and Bosnia and Herzegovina namely: Dahna, Ponor Kovači, Tamnica, Topla peć, Velika peć, Tounjčica, Veternica, Lisičja špilja, Listvača, Miljacka II, Pražina pećina, Čakovac, Horvatova špilja, Špilja na Crvenom jezeru, Hukavica, Rogovac and Kotluša. Cave Veternica (Lacković et al., 278) is formed on isolated mountain karst area in the northern part of Croatia. Other caves are part of the Dinaric karst area (Mihevc et al., 2010).
Centropyxis bipilata has not been detected on the surface habitats close to the researched caves, even though surface habitats have not been systematically studied yet.
Differential diagnosis: Frenopyxis bipilata resembles C. laevigata in general view. However, in C. laevigata the inner and the anterior wall of the shell are connected by a broad flange covered with thick amorphous scales and the test is larger (70-150 μm, Penard 1902). Schönborn et al. (1983) described a similar species from different locations in Germany as C. laevigata, but their species always had three struts. F. bipilata differs from C. malvinensis that has the same size and apertural border but two lateral flanges in the middle part (Vucetich, 1975). It can also be distinguished from C. obscurus that doesn’t have a clear rim around the aperture nor any struts (Chardez, 1990). It differs from C. delicatula that is smaller, has a variable number of 3-5 struts and no pronounced apertural rim (Penard, 1902).