Diagnosis. Superficial furrows of more or less straight path, either occurring in isolation or in dense populations. In both cases the furrows are distinct, although very shallow. Ichnospecimens in dense groups branch in Y-shaped patterns; isolated examples branch at acute, almost orthogonal angles (Pl. 2, figs 7–8).

Description. C. tenuis is most easily identified by the calcareous remains of the tubular shell of the producers, which are often preserved. When shell remains are absent, the path of the trace is defined by one or several shallow furrows that smooth out the microtopography of the substrate, erasing any surface ornamentation (Pl. 2, figs 7–8). Furrow width ranges from 26 to 40µm and their depth scarcely reaches 2µm. Uneroded marginal areas are parallel to each side of the furrow; the width of these areas is usually greater than the furrow itself, ranging from 20 to 66µm (Text-fig. 2D–E; Pl. 2, figs 5–6). Where ichnospecimen density is relatively high (Pl. 2, fig. 5), the furrows branch at acute angles between 40 and 45 degrees and are organised into more or less parallel series. When density is much lower, furrows branch off at almost orthogonal angles (Pl. 2, figs 7–8).