Fig. 6 in An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia

Fig. 6. Helcionellid mollusc Parailsanella sp. 1 internal moulds from lower Cambrian Emyaksin Formation, Bol'shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/36.7. A. SMNH Mo 167656; close-up of Fig. 5C 2; A 1, pits on the surface of internal mould; A 2, close-up of A 1. B. SMNH Mo 160416; close-ups of Fig. 5D; B 1 , lateral view, polygonal texture on the mould; B 2 , polygons, A B close-up of B 1. Scale bar: A 1, 120 μm; A 2 and B 2, 30 μm; B 1, 300 μm. margin and continuous with the pegma-like structure (Figs. 9C, 11C). Remarks.—The surface of the moulds is covered with 10–40 μm wide, blunt tubercles and bears reticulate microornamentation best seen on the convex parts (folds) of the mould. The tubercles tend to be situated in comarginal rows, similar to what occurs in Mellopegma (Vendrasco et al. 2011b). Where concentric folds are developed, the tubercles are concentrated D along their surface. Reticulation is formed by shallow concave C polygons, ca. 20 μm wide. On the surface of concentric folds, the polygons become deeper and more variable in size, with a diameter typically ranging from 2–10 μm (Fig. 10G). The microornamentation flattens out in the subapical area. In addition to the polygonal texture, some specimens of Mackinnonia show imprints of a laminar shell microstructure in the concave bands between folds on the internal mould (Fig. 10C, G). Fig. 7. Helcionellid mollusc Parailsanella sp. 2 internal moulds from low- er Cambrian Emyaksin Formation, Bol'shaya Kuonamka River, Anabar As demonstrated by Runnegar (in Bengtson et al. 1990: fig. Uplift, Siberia; SMNH Mo 167658, sample 7/70; lateral (A), sub-apical 159g), the troughs on internal moulds of Mackinnonia corre- (B), and oblique upper (C) views. D. Close-up of A, showing pits on the spond to areas where the shell was thicker, and where the inner surface of internal mould. Scale bar 1200 μm, except D, 300 μm. layer of shell microstructure would be preserved, whereas the