The text in Müller's work is reproduced in Fig. 2. My translation of the Latin text is: " D. parvulus, pronotum black, anteriorly yellow; elytra with a shortened stria; legs brownish black."
Müller's description fits almost all species of Hydroglyphus and Bidessus occurring not only in northern Europe. However, since Müller's work deals with Scandinavian species, these should be either Bidessus unistriatus, B. grossepunctatus, Hydroglyphus geminus or H. hamulatus (Gyllenhal, 1813) (see e.g. Nilsson & Holmen 1995). The type locality of parvulus is Norway and Denmark (see Nilsson & Hájek 2019b: 104). The identity of the taxon cannot be checked since an insect collection of O.F. Müller never did exist or is totally lost (Horn et al. 1990b: 273; Evenhuis 1997: 555; Nilsson & Petrov 2006: 161; see also Nilsson & Hájek 2019b where under all taxa of O.F. Müller is given " syntypes lost").
I know of about 90 works in which the name parvulus is used (in combination with several different generic names and authors). Only in 12 of these works is it used as a valid name with author O.F. Müller, one of them is Crotch (1872: 204) who gave Goeze's and Schrank's names as synonyms of parvulus. Heyden (1880: 54) was the first who included the name as valid in combination with the generic name Bidessus Sharp; additionally, he gave geminus as a junior synonym. Baudi (1889: 34) and Ádám (1996) are the last authors who treated Müller's name as valid. Otherwise the taxon is mostly treated as synonym of unistriatus (with authors Schrank, Goeze or others, in several cases also as "sensu" name).
Thus, we have the problem that parvulus O.F. Müller is an available name (although a nomen dubium) and possibly a senior subjective synonym of unistriatus Goeze and also of unistriatus Schrank, and might be even a senior subjective synonym of Dytiscus geminus Fabricius, 1792. Since parvulus is an almost forgotten name, the use of this name instead of either Goeze's or Schrank's or Fabricius' name would considerably threaten the stability of the nomenclature.
It would be easy to give the younger name precedence over the older name by application of Article 23.9 of the ICZN (1999) —if Ádám (1996: 18, 60) would not have used it as valid name. Special problems arise because he even designated it as type species of Bidessus Bedel, 1881 (see also Bousquet & Bouchard 2018).
It must be also mentioned that Müller's name was already involved in another nomenclatural problem. Motschulsky (1853b: 77) published a new species from Alaska with the name Dytiscus parvulus. The homonymy was removed by J. Balfour-Browne who replaced Motschulsky's name with Dytiscus alaskanus J. Balfour-Browne, 1944: 356, both names being now objective synonyms.