Scholars divide the great bulk of the Slavic languages into two branches […] The south-eastern or eastern division contains the Russian, Bulgarian, and Illyrian languages1855, “Europe”, in Encyclopedia Britannica, Eighth Edition, volume 9, page 391:
The Slavic dialect used was examined by a very gifted poet, Gion Palmotich (1606–1657) , who attempts to identify the Dalmatian or Illyrian language (i. e., Serbo-Kroatian) with the Czech of Bohemia, which had been recognized by Charles the Fourth, in his Golden Bull (1355), as necessary for imperial princes to learn.1919, Paul Rankov Radosavljevich, Who are the Slavs? A Contribution to Race Psychology, pages 198–199:
1 more example
The preface contains a defense of the Illyrian language and a list of the major Slavic “dialects” (Russian, Polish, Bohemian, Illyrian).2016, Edward Stankiewicz, Grammars and Dictionaries of the Slavic Languages from the Middle Ages Up to 1850: An Annotated Bibliography, page 80:
Illyrian
noun
1
An inhabitant of ancient Illyria.
2
An inhabitant of Roman province of Illyricum (Illyro-Roman).
3
Obsolete A South Slav, especially a Croat, used in 17th-19th centuries.