Description. - Shell rather small, high-spired, cone-shaped, slender, spire about one half total height.
Protoconch smooth, globular, hyperstrophic, approximately diverging 40-45° from teleoconch axis; embryonic whorl exposed (the first whorl completely visible), multispiral consisting of 2,5 whorls; protoconch diameter 570 µm.
Teleoconch with rounded whorls, and distinct suture. Pattern of regular spiral cross¬ing weaker and slightly arcuate axial ribs (24 on last whorl), intersecting at approximately right angles forming nodules. Spacing of the axial ribs regular, the crossing of spiral and axial ribs form a well ordered arrangement of rectangles, the bigger created between the two lowermost spiral ribs. Spiral sculpture consisting of four ribs ("2+2" pattern), the first uppermost and the middle third one stronger, the second on the upper part of the whorl and the fourth lowermost weaker; the third rib is the most prominent. Four additional fine spiral threads are visible among the major spiral ribs starting from the seventh whorl; in the last whorl, a fifth spiral rib almost fully covered, barely visible at the suture and exposed on the base of shell, forming a clear double keel. Eight fine spiral threads are visible at the basal area, umbilicus closed. Colour of the dead-collected holotype shell whitish. Operculum unknown.
Remarks. - This new taxon shows the typical mathildid "2+2" pattern , it is similar to M. letei but it can be separated on the base of some shell diagnostic features: larger size, lower TW/H (in the adult specimens), protoconch diameter (PD) smaller (570 µm versus 640 µm), spiral rib arrangement different in relative intensity and spacing of ribs, spiral threads at the basal area more numerous (8 versus 5-6), white colour. Furthermore, the spiral ribs of M. bieleri show less marked nodules than the ones of M. letei. Mathilda bieleri is very similar to Mathilda spec. A, the differences between these two shells are discussed below. For a comparison with the sculpture of the other species see the remarks on M. letei. This new taxon was collected on a bathyal bottom under investigation (Smriglio & Mariottini, 2002), which is dominated by deep-sea coral communities (biocoenosis CB). The analysis of the sediment dredged from this marine site has yielded many fragments of the azooxanthellate corals Desmophyllum cristagalli Milne Edwards & Haime, 1848, and Dendrophyllia cornigera (Lamarck, 1816).