Selonian (Selian) language
The documents from the 13th - 15th centuries name one Baltic tribe who used to be living in east Latvia and north-east Lithuania, speaking a language that Russian manuscripts called Selonian. This nation was small, and is included in the so-called "external" subgroup of Baltic languages - its speakers neighboured with Slavs who were much more civilized and numerous at that time and that's why influenced Selonian strongly.

When the Great Lithuanian Principality was established in the 13th century, all minor tribes and languages of Lithuania started to suffer influence also from Lithuanian, and so in the next century Selonian was completely assimilated and became extinct. In 1375 Teutonic chronics call these lands populated by Lithuanians.

Nowadays according to the place names remaining from that times in the region we can say that the tongue looked quite like Lithuanian, had many borrowings from Slavic and Finno-Ugric languages, was rather conservative in structure. The substratum from Selonian is sometimes seen in some east Lithuanian and Latvian dialects.

The region's toponims show the language had z, s sounds instead of Lithuanian and z', then c [ts] instead of Lithuanian palatal k. The place name keep Selonian frequent suffixes -aja-, -uoj-, -as-, -es- and others. Nowadays dialects of this district of Lithuania have some signs of Selonian as well: the change of the open e. into a (Lith. > dada).