The dimensions of the apex is d < 0.09 mm. The length of the shells varies, 2.3-4.7 mm. The colour of the great majority of the shells is white or somewhat yellowish. In such shells, the upper whorls have the same colour as the lower part of the shell, except for the first half whorl which may be tinged with lilac or purple. Between the ribs, narrow yellowish brown longitudinal colour lines can often be seen, which may be broken into two or, rarely, three parts as shown in fig. 9, and which continue on the base of the shell. Shells from the east coast of the Adriatic Sea, however, rarely show this type of pattern. Instead, they may be covered by interrupted brownish spiral lines. I saw similar shells from Naxos, Greece (Cad). There may be two or three brown or purplish spots at the edge of the aperture of shells of R. similis. The entire aperture is not infrequently tinged with purple. As in R. dolium Nyst, 1843, brown specimens do occur occasionally. Shells with one or two brown colour bands, or with part of the whorls coloured brown are also known. The lower two or three whorls of the great majority of the shells are uniformly covered with punctate spiral striae, which also cover the base of the shell. In ribless shells this sculpture may be weak or absent. Such shells may be easily mistaken for R. parva forma interrupta (Adams, 1798). Occasionally, one finds ribbed shells without spiral sculpture. The great majority of the shells is ribbed. Usually, there are 11.5-14 terminal ribs per whorl.

Bucquoy, Dautzenberg & Dollfus(1884: 265) wrote: "R. similis est fort difficile a delimiter d'une maniere satisfaisante. Certains individus se rapprochent en effet du R. subcostulata Schw., d'autres des R. melanostoma Req. et R. Lia Benoit." For two reasons, my experience is quite different. In the first place, I had the opportunity to examine much more material, and from a more extensive area than the above authors probably did. In particular this refers to samples from Portugal, S. Spain, Sicily, the Adriatic Sea and the Aegean Sea, which played an important part in this investigation. Secondly, I paid much attention to the dimensions of the apex. In my experience, R. similis is a rather constant species, sharply delimited from the other species by a combination of characters as regards sculpture, apical dimensions and colour. The examination of many hundreds of specimens from the whole Mediterranean and from Portugal has convinced me that R. similis always has the smaller type of apex.

R. similis is washed ashore in great quantities from Lagos, S. Portugal, to Israel. In my limited experience the species is somewhat more scarce in the eastern part of the Mediterranean than in the western.