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COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In the male elasmobranchs the pelvic fin is divided into two lobes, 

 the medial, the so-called clasper (mixipterygium) being the longer and 

 narrower. This is used in copulation and is supported by thespecialized 

 terminal radialia of the metapterygium. 



In other ganoids and in teleosts the skeletal parts are more or less 

 ossified, the basalia more numerous than in the higher elasmobranchs 

 and are shortened and more closely associated with the girdles, while 



the numerous radii form most of the 

 skeleton of the fin itself. It is not un- 

 common for the anterior element of the 

 pectoral fin to form a strong defensive 

 spine, not infrequently connected with a 

 poison gland. In some teleosts, e.g., 

 eels, the pelvic fin may be lacking. 

 The fins of the dipnoi are easily under- 

 stood by comparison with a biserial fin 

 like that of Xenacanthus (fig. 124). The 

 axial part has been elongated and in 

 Ceratodus it bears biscerial radialia, 

 while in Protopterus and Lepidosiren 

 only the axis persists. 



FIG. 125 Cartilage skeleton of Embryology tells little as to the primitive 

 shoulder girdle and left pectoral fin of 



larval Polypterus, after Budgett, bvf, condition of the ichthyopterygmm, for in the 

 foramina for blood-vessels; c, cora- procartilage stage the condensation of mesen- 

 ^p^guiS;^, S^giZ; V for the ^eleton of , he fin forms a con- 

 pro, protopteryigum; r, developing tinuum which later becomes broken into the 

 radialia; s, scapula. separate parts (fig. 125). 



The legs (chiropterygia) of all tetrapoda are essentially alike (fig. 

 1 08). Each consists of several regions, comparable in detail with each 

 other. The proximal is the upper arm (brachium) or thigh (femur) 

 containing a single bone, the humerus or femur in the fore and hind 

 limb respectively. The next region, the forearm (antebrachium) 

 or shank (cms), contains two bones, a radius or tibia on the preaxial 

 and an ulna or fibula on the postaxial side. Next follows the podium, 

 the hand (maims) in front, the foot (pes) behind, each consisting of 

 three portions. The basal podial region, the wrist (carpus) or ankle 

 (tarsus) consists of several small bones; the second division (metapo- 

 dium) is the palm (metacarpus) or instep (metatarsus) and lastly 

 come the fingers or toes (digits) , each digit consisting of several bones. 



