Sea-urchins (endocyclic echinoids) play a significant role in stratigraphy of Cretaceous sediments in TurkmeniStan. They are numerous and versatile at genus and species levels, are typical of various parts of lower Cretaceous sediments. Sea-urchin shells have been selected from two section of the Bolshoy Balkhan edge - Kazanzhabu-run, Taschlydere ravine. Sediments in those sections are represented with different types of limestone: dolo-mitic, pelitomorphic and organic-fragmental with dolomite and malm interlayers. All the shells were found at the Goteriv layer bedding (lower part of the Kazanzhaburunskaya series). 10 species of the Toxaster genus have been determined and described, two of them probably being new. Despite the discoveries of numerous specimens of the same species, a number of differences at the species level have been noted in both sections (several shells of certain species have been found in one section and have not been revealed in the other). The described sea-urchins fauna, particularly such species as Toxaster retusus and Toxaster holas-teroides, enables correlation of the Bolshoy Balkhan sediments with the sediments of the neighbouring regions (Kopetdag, etc.) and even with the Goteriv sediments of foreign countries (Western Europe, Morocco, Algeria). Toxaster specimens show clear stratigraphic attachment to both layer and sublayer. Thus, in the above mentioned layers specimens typical of the Goteriv sediments in Portugal, France, Switzerland and other countries have been found. This proves the existence of a common paleobasin in the Lower Cretaceous period, which was the centre of Toxasteridae sea-urchins development and expansion. It is necessary to mention the fact, that there have also been found specimens of sea-urchins, typical only of the Bolshoy Balkhan sections and never encountered in other sections of the same age. This is, probably, concerned with particular pecularities of the Bolshoy Balkhan paleobasin.
