Dictionary of Ichthyology  

mesopterygium (plural mesopterygia) = the median, elongated basal cartilage of the pectoral fins in Elasmobranchii, Holocephali and primitive Teleostomi. ferent sizes, and may have a whistle, bell or gong.
Shoulder girdle and proximal part of pectoral fin of Raja, g = right half of girdle, ms = mesopterygium, mt = metapterygium, p = protopterygium, r = radii of fin, from Kingsley's "Textbook of Vertebrate Zoology, 1906. Photograph by Brian W. Coad.

           Canadian Museum of Nature