Brachystomia with moderately prosocline growth lines, and a turreted shell shape with quite convex whorls and deep sutures. Rounded aperture. Protoconch of type C in Van Aartsen typology (intorted) of less than one whorl and with a clear demarcation of the beginning of the first teleoconch whorl. Soft parts: Head foot complex white with numerous small yellow pigment spots scattered over foot and tentacles, tentacles triangular with tentacular pads, mentum slightly expanded at the tip, eyes fairly large and not particularly close together. Pigmented mantle organ, yellow blotch with dark brown interrupted edges, masses of white 'bubbles' above and an oblong custard coloured gland further up. Operculum: flat with only a slight thickening under the central part, no clear indentation for the columellar tooth.

AS Odostomia scalaris
Habitat: it lives in the infralittoral zone and it is host of many opistobranches and lamellibranches, of Mytilus edulis mainly. Distribution: it can be found all over the Mediterranean. Uncommon.
Notes: species rissoid in aspect with H/L ratio lower than 2. Whorls rather convex, suture evident. Surface dull and sometimes crossed by a spiral sculpture weak. Sometimes it is visible a slight concavity below-suture. Growth lines prosocline. Aperture oval in form with one small columellar tooth, umbilicus evident. Protoconch type C (with internal nucleus). Among the synonyms of this species we remind: O. nitida, O. rissoides, O. alba, O. rissoiformis. Average measures of adult specimens are less than 3 mm in height.

B. scalaris seems to be a typical shallow water species, rarely found deeper than 15 m in our waters. It has most frequently been encountered as an ectoparasite of Mytilus edulis, but has been reported from a number of other molluscan hosts as well as free living in shallow water (Ankel & Christensen 1963, Rasmussen 1973, Fretter et al. 1986). In my studies (Hoisater 1989), the species was found in samples dominated by Limaría hians and Modiolus, but also in samples of Pomatoceros reefs, and finally from haptera and stipes of Laminaria hyperborea in semi-exposed and fairly protected areas. In the first and last of these it was invariably the most numerous pyramidellid species. Like B. eulimoides primarily a mollusk-feeder, but my studies indicate that it occasionally co-occurs with other pyramidellids on Pomatoceros reefs.

Rarely recorded from Norway before 1986. Norman (1879) reports it from Raunefjorden and Osterfjorden and cites earlier reports from Oslofjorden (from Jeffreys 1870). G.O. Sars (1878) records it from Oslofjorden, the southern coast and the west coast. In my material 13 samples with 34 specimens from Skagerrak, and 705 specimens from the Espegrend area. Further north 33 specimens and 39 shells, the northernmost empty shell from southwest of Bodo (67°15'N9 50-20 m, shell gravel, slag and small stones). A sample from a shallow7 water algae station (containing more than 1200 Bittium reticulatum) at Floan, bay southeast of Stamnes (64°29,N, 3-10 m, soft bottom with Laminaria saccharina, Chorda filum and other algae) contained at least 30 B. scalaris. This is thus at present the northern limit for the species (based on living material). Outside Norway it is known from Sweden and Denmark, even the western part of the Baltic Sea, all around the North Sea, the western and southern coasts of Ireland and the British Isles, further south along the Atlantic coasts of Europe (Ankel 1936) and the western Mediterranean (Peñas et al. 1996). Van Aartsen et al. (1998) extend the distribution to Mauritania, and also cite earlier records from Madeira, the Selvagens archipelago and the Azores.

DISTINCTIVE FEATURES:

• Shells with a clear tooth on the columella.
• Shell-surface with signs of (weak) spiral striature
• No (spiral) ribs.
• Inside of the outer lip smooth.
• Embryonic whorls of type B, somewhat tilted.
• Growthlines more or less vertical.
• Shell-surface somewhat dull in appearance.
• Shells of rissoid shape.
• Height/ Breadth ratio <2.0