Dictionary of Ichthyology  

protopterygium = one of the three basal cartilages of the pectoral fin in the the sharks and rays along with the protopterygium and mesopterygium, both q.v.

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