R. aartseni is so similar to R. lia torquilla that it is not very well possible to draw a sharp border-line between both species. R. aartseni differs in the following characters: (1) the greater slenderness; (2) the more expanded aperture; (3) the somewhat higher number of ribbed whorls, over 4,5 in most shells; (4) the low number of terminal ribs per whorl, rarely more than 9; (5) the colour, longitudinal zigzag colour lines seem to be absent among R. aartseni; instead, straight colour lines can be seen between the ribs of some specimens; (6) the presence of punctate spiral striae on the lowermost part of some shells.

R. aartseni seems to be restricted to the Golfe de Gabes, Tunisia.

Dr. J.J. van Aartsen drew my attention to the fact that the name Rissoa monterosatoi Pallary, 1906, is preoccupied by Rissoa (Alvania) monterosatoi P. Fischer, 1877. Therefore, I now propose the replacement name R. aartseni, after my good friend, whose vast knowledge of, and whose contributions to, the European marine malacology I greatly appreciate.
Because, in my experience, the distribution of R. paradoxa is still limited to Tunisia, where the slender form even seems to be restricted to the Golfe de Gabes, I believe the sample to have been collected in the latter area. The remainder of the sample consists of shells which are very similar indeed to the bleached shells of R. aartseni. There are, however, a few small differences. The ribs of the fresh shells are somewhat flatter and less prominent. Moreover, punctate spiral striae are present in a majority of the fresh shells, and may even cover the whole body whorl. One of the fresh shells has the chess-board colour pattern. All others show straight brownish colour lines between the ribs. For the time being I consider them conspecific with R. aartseni. As in R. decorata, the colour of the uppermost whorls of the fresh shells scarcely differs from that of the remainder of the shell, though in part of the shells it tends to be a light pink. In combination with the colour patterns of the lower whorls and the low number of terminal ribs per whorl, i.e. 7-9, rarely 10, this suggests a close relationship between both species. They, however, clearly differ in the slenderness of the shells and the aperture, which is expanded in R. aartseni.