The functional morphology of the fourth chamber of the fish heart,, the bulbus arteriosus, differs in teleosts and elasmobranchs. In elasmobranchs, this chamber is referred to as the conus arteriosus because its wall is made up of myocardial tissue in addition to elastic tissue. AS a result of this feature, the function of the conus is very specialized. We report the basic ultrastructure of the wall of the conus arteriosus in the elasmobranch Scyllium stellare. The wall consists of three layers: the outer epicardial, the middle myocardial, and the inner endocardia1 layer. The outer layer is composed of the visceral pericardium consisting of cuboidal mesothelial cells, beneath which numerous collagen fibers and blood vessels occur. The middle layer is constituted of myocytes that are continuous with those of the ventricular wall, thus assuring the electrical and mechanical coupling between the ventricle and the conus. The inner layer exhibits a continuous lining of squamous endothelial cells containing spheroidal electron-dense granules. The subendothelium of the ventral surface is characterized by chromaffin cells endowed with numerous osmiophilic granules associated with myelinated and unmyelinated nerve endings. These chromaffin cells have a glomus-like appearance. These findings are discussed in relation to the role of the conus as modulator of the hemodynamic events occurring between the heart and gill vasculature. '71; Johansen and Gesser, '86).