Abstract
An additional study of the cephalic shield and trunk scales of the holotype of Ungulaspis arctoa Afanassieva et Karatajūtė-Talimaa has shown that the exoskeleton surface is composed of dentin of at least two generations. Based on new data, the exoskeleton of Ungulaspis is reconstructed. A model for ontogenetic development of the exoskeleton of Ungulaspis arctoa is proposed. The superposition growth of osteostracan exoskeleton is described in detail for the first time using macro-rather than microremains. Using Osteostraci as an example, the existence of two (bipolar and unipolar) types of vertical growth of the exoskeleton in some groups of lower vertebrates is shown. The taxonomic position of Ungulaspis arctoa in the system of Osteostraci is discussed.
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