(D) _Appendicular Skeleton._--The skeleton of the free part of the limb is attached to the limb girdle which lies embedded in the musculature of the body. Each limb girdle is probably to be looked upon as consisting, like the skeleton of the visceral arches, of a pair of lateral half-hoops of cartilage. While in _Pleuracanthus_ the lateral halves are distinct (and segmented like the branchial arches), in living Selachians generally the two halves are completely fused ventrally with one another. The part of the girdle lying dorsal to the articulation of the limb is termed scapular in the case of the pectoral limb, iliac in the case of the pelvic, while the ventral portions are known respectively as coracoid and ischio-pubic. [Illustration: FIG. 17.--a, Skeleton of pectoral limb of _Pleuracanthus_. (From Gegenbauer, after Frisch.) b, Skeleton of pectoral limb of _Acanthias_. (After Gegenbauer.)] In most Teleostomes the primitive pelvic girdle does not develop; in the Dipneusti it is represented by a median unpaired cartilage. The skeleton of the free limb is probably seen in its most archaic form amongst existing fishes in the biserial archipterygium of _Ceratodus_ (fig. 16). This is indicated by the relative predominance of this type of fin amongst the geologically more ancient fishes. The biserial archipterygium consists of a segmented axial rod, bearing a praeaxial and a postaxial series of jointed rays. In _Protopterus_ and _Lepidosiren_ the limbs are reduced and the lateral rays have less (_Protopterus_) or more (_Lepidosiren_) completely disappeared. [Illustration: From Budgett, _Trans. Zool. Soc. London_, xvi, part vii. From Wiedersheim's _Verg. Anat. der Wirbeltiere_, by permission of Gustav Fischer. FIG. 18.--Skeleton of Pectoral Limb of _Polypterus_. a, 30 mm. larva. b, Adult.] [Illustration: From Wiesdersheim's _Verg. Anat. der Wirbeltiere_, by permission of Gustav Fischer. FIG. 19.--Skeleton of Pectoral Fin of _Amia_.]