CLASS OF FISHES. 341 



the vertebrae, aiid articulate by the other extremities with 

 the radii of the median fins (r) ; these radii vary much 

 in different fishes: sometimes they are only ossified at the 

 base, formed afterwards of a number of small articulations. 

 These last are called soft or articulated radii ; they always form 

 the caudal fin (Fig. 307), and sometimes there are no others. 

 480. The lateral fins, representing the limbs, are ter- 

 minated by radii similar to the dorsal, and analogous to the 

 fingers ; in the pectoral fin, four or five small flat bones (Fig. 

 309 cd) represent the carpus, and these are supported by two 

 flat bones (ab), the radius and the ulna. The osseous girdle 

 supporting these is composed of a series of three bones, of 

 which one (K) represents the humerus, a second (6) the sca- 

 pula, and a third, composed of two pieces, may be called the 



Fig. 311. Dorsal Fin. 



coracoid (co). The posterior limb (Fig. 307) is less complex ; 

 the radii of the ventral fin are supported only on a single bone. 

 481. In cartilaginous fishes, such as the skate and the 

 shark, the skeleton somewhat resembles that of the tadpole. 

 The cranium is without sutures, and is composed of a single 

 piece, but modelled and pierced like the cranium of a common 

 fish. The upper jaw is formed of pieces analogous to the 

 palatine bones and to the vomer; the maxillary and inter- 

 maxillary bones either do not exist or are merely rudimentary. 

 The lower jaw is also formed of a single piece on each side, 

 and the opercular apparatus is wanting. The vertebral column 

 is often formed of a single tube, pierced on each side for the 

 passage of the nerves, but not divided into distinct vertebrae ; 

 and the gelatinous-looking substance acting as a ligament to 

 connect the vertebrae to each other, often forms a continuous 

 cord. The arrangement of the bones of the shoulder, pelvis, 



