116 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. ‘Thisis 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. 
Fic. 125.—Cartilage skeleton of i imiti 
digildenpinisiau ifepe ton linet Embryology tells little as to the primitive 
larval Polypterus, after Budgett, buf, condition of the ichthyopterygium, for in the 
foramina for blood-vessels; c, cora- procartilage stage the condensation of mesen- 
coid; cf, coracoid foramen; mes, 
mesopte eyesiines met, metapte ae chyme for the skeleton of the fin forms a con- 
pro, protopteryigum; 7, 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. 
108). 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 (crus), 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 (manus) 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, 
