Heterlimnius Hinton 1935 : 178

Bukejs, Andris, Alekseev, Vitalii I. & Jäch, Manfred A., 2015, The riffle beetles (Coleoptera: Elmidae) of the Eocene Baltic amber: Heterelmis groehni sp. nov. and Heterlimnius samlandicus (Bollow, 1940) comb. nov., Zootaxa 3986 (4), pp. 452-460 : 453

Heterlimnius Hinton 1935: 178

Palaeoriohelmis Bollow 1940: 117 syn. nov. (type species: Palaeoriohelmis samlandica Bollow, 1940 )

The genus Palaeoriohelmis is herewith synonymized with the genus Heterlimnius on the basis of the following morphological characteristics: (1) subovate body with plastron mainly on ventral surface; (2) filiform 11- segmented antennae with symmetrical antennomere 11; (3) pronotum with basal sublateral longitudinal grooves and with very shallow basal transverse depression; (4) prosternum produced anteriorly; (5) elytra with uniform, slightly convex, not carinate intervals; (6) tarsal claws simple, curved, long.

Bollow (1940) compared Palaeoriohelmis with two extant West Palearctic genera: Lathelmis Reitter, 1883 [= Limnius Illiger, 1802 ] and Riolus Mulsant & Rey, 1872 , stating that the new genus was related with Riolus on account of the lacking “eingeschnittenen Dorsalstreifen des Halsschildes” [grooves of the pronotum]. However, examination of the holotype revealed that a pair of basal sublateral longitudinal grooves is indeed present in Palaeoriohelmis .

It should be mentioned here that Heterlimnius cannot be distinguished reliably from Optioservus . These genera are very similar in general morphology and distribution (both occur in the East Palearctic, Nearctic and northern Oriental Regions), and members of both genera have been historically confused with one another. At present, 14 species are recorded in Heterlimnius , and 19 in Optioservus . The only possibly reliable character to distinguish the two genera is provided by the ventral sclerites of the larval meso- and metathorax ( Kamite 2013), but larvae of several species are still unknown. Quite possibly, these two genera will have to be synonymized following comprehensive molecular studies, which have not been carried out so far.