Ischnochiton abbessi

Ischnochiton abbessi (Cherns & Schwabe, 2019) Fig. 63 Chaetopleura abbessi Cherns & Schwabe, 2019, p. 8, figs 6(J)–(N). Ischnochiton abbessi; Dell’Angelo et al. 2018a, p. 26, figs 5S–U, 6A–C. Type material. Holotype, ZSM Mol 20071427, intermediate valve, width 9.2 mm (Figs 63F–H). Type locality. Abbesse (Aquitaine, France), see rectification for the locality in Dell’Angelo et al. (2018a: 26, not Abbesse, Bourgogne). Type stage. Upper Oligocene (Chattian). Material examined. Lower Miocene: France (Burdigalian): Gamachot: MHNBx 2017.9.2, intermediate valve, width 7.8 mm (Figs 63A–E). Description. Head and tail valves unknown. Intermediate valve broadly rectangular, large, broad and short (W/ L = 2.74–3.60), gently rounded in anterior profile, moderately elevated (H/W = 0.30–0.34), anterior margin straight across jugal area, rounding convexly outwards, side margins rounded, posterior margin nearly straight, narrow triangular, slightly elevated rounded jugal fold, lateral areas narrow, delineated by low diagonal beaded ridge and change of ornament. Tegmental sculpture consisting of coarse beaded ribs strong outside worn JA, longitudinal on CA, radial on LA. Apophyses short, barely projecting beyond the anterior margin, rounded, well separated across broad jugal sinus, ventral surface smooth, short wide ribbon-like apical area across posterior margin, wider and anteriorly triangular in apical region, no insertion plates visible but indications of slit rays. Remarks. Ischnochiton abbessi (Cherns & Schwabe, 2019) is known only for two intermediate valves, the first originally described from the Oligocene of Abbesse (as Chaetopleura) and the second from the Burdigalian of Gamachot by Dell’Angelo et al. (2018a), which assigned the valves to the genus Ischnochiton, on the basis of strong similarities to species included in this genus. The characters of the valve from the Burdigalian of Gamachot agree with those from the late Oligocene of Abbesse, with only some slight differences in the shape (W/L = 2.74 vs. 3.60, and H/W = 0.34 vs. 0.30), but the studied material is really scarce and does not allow to define the intraspecific variability of this species. Comparisons. See Tabs 9, 10 for a comparison with the Ischnochiton spp. considered in the present study. Distribution. Upper Oligocene: northeastern Atlantic (Chattian): Aquitaine Basin, France: Abbesse (Cherns & Schwabe 2019). Lower Miocene: northeastern Atlantic (Burdigalian): Aquitaine Basin, France: Gamachot (Dell’Angelo et al. 2018a).