. The elasmobranch fishes . A B Fig. 81. A aud B. Stages in development of the pectoral fin of Scylliuyn canicula. (From Balfour.) , mesopterjgium; , metapterygium; , propterygium; ra., radial. Few calcified rays may be present as in Gal ens, or they may be more numerous as in Heterodontus (fig. 79a) and Rhinohatis (fig. 79b) . In Alopias more than twenty rays are present. So characteristic are the above types of calcification that Hasse has used them as a basis for classification. Under such a classification exceptions must be made, however, for many variations are to be found

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. The elasmobranch fishes . A B Fig. 81. A aud B. Stages in development of the pectoral fin of Scylliuyn canicula. (From Balfour.) , mesopterjgium; , metapterygium; , propterygium; ra., radial. Few calcified rays may be present as in Gal ens, or they may be more numerous as in Heterodontus (fig. 79a) and Rhinohatis (fig. 79b) . In Alopias more than twenty rays are present. So characteristic are the above types of calcification that Hasse has used them as a basis for classification. Under such a classification exceptions must be made, however, for many variations are to be found. Appendicular Skeleton SKELETON OF PAIEED FINS AND OF FIN GIEDLES PECTOEAL FIN SKELETON The skeleton of the pectoral fin, as noted in Heptanchus, consists of a more or less horizontal framework of cartilage attached to a vertical girdle. The car- tilages making up the framework of the pectoral fin itself are: (1) a set of basal cartilages from which projects (2) a series of median cartilaginous

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