The genus Kozlowskiellina Boucot, 1957, which comprises about nine species, has a stratigraphic range from Wenlock (Middle Silurian) up to the Upper Emsian (Lower Devonian). In this paper, several characters are described: the micro-ornamentation, the internal characters of the pedicle valve, and the interior of the brachial valve. With respect to these three characters, there is a great diversity within the genus, especially in the pedicle valve, some of the species having dental plates and others lacking these structures. In addition to the description of the micro-ornamentation, a functional interpretation of some features of this ornamentation is given. Because of the diversity, it seems impossible to describe the genus with one chosen type species. Therefore, a historic interpretation is given that represents the essence of the genus. A genus is a group of species which are historically closely related; a description of a genus is the description of the morphological history of that genus. Consequently, an attempt has been made in this paper to define the historic relationship between the different species within the historic group of the genus Kozlowskiellina.