Dictionary of Ichthyology  

metapterygium (plural metapterygia) = the posterior and innermost basal cartilage in the paired fins of Elasmobranchii and Holocephali. It is usually the longest basal and is exceptionally long in males where it is distally modified into a clasper, 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