The aggregated section of Middle Cambrian at the Khorbusuonka River, northeastern Siberian Platform, is described. The aggregated sequence is compiled on the basis of investigations at four bank outcrops. The findings of trilobites at different stratigraphic levels allowed us to identify the intervals comparable to regional zones of the Amgan and Mayan stages (Middle Cambrian). New findings and analysis of stratigraphic distribution of trilobites helped to specify the position of the Middle Cambrian lower boundary. The lower boundary of the Mayan Stage is drawn by the first appearance of Corynexochus perforatus in the section, 9 m from the base of the Yunkyulyabit-Yuryakh Formation. Trilobites of Cyclolorenzella sp. and Kuraspis sp. found in the upper Tyussala Formation are characteristic of the boundary layers of the Middle and Upper Cambrian. It was supposed earlier that the Upper Cambrian part of the sequence began in the overlying Lapar Formation; however, the aforementioned findings suggest that the Middle Cambrian lower boundary can be located within the limits of the upper Tyussala Formation.