10 STRrCTIJEE or THE PAIEED FINS OF CERATODUS, 



higtly specialised than in even the adult Dog Fish or Eaj. In 

 these the muscles which move the fin are flat plates made up of 

 coarse fasciculi radiating from the limb arch to the bases of the 

 fin rays. 



In GeratocliLs there is a strong extensor muscle arising from the 

 shoulder-girdle and inserted along the dorsal surface of the basal 

 joint, the next joint, and the first two joints of the central axis ; 

 and a flexor muscle with a similar arrangement but interrupted 

 opposite the distal end of the basal joint, the distal portion of 

 its fibres chiefly having their proximal attachment with the 

 tubercle at the distal end of the ventral surface of the basal 

 cartilage. Along the axis of the fin both on the dorsal and the 

 ventral surfaces run a series of interrupted muscular fasciculi 

 which pass between adjacent joints of the axis and the basal 

 joints of the rays, and passing obliquely outward from those are 

 series of fibres connected externally with the bases of the fin-rays. 

 The front portion of the extensor turns round the anterior margin 

 so that it acts to some extent as an abductor. Beneath it are two 

 muscles, one arising from the shoulder girdle and inserted wholly 

 into the basal cartilage ; and the other arising from a prominent 

 tubercle near the distal end of that cartilage, its fibres passing in. 

 a radiating manner to be inserted into the next joint and the two 

 basal joints of the anterior axis ; this muscle must act as a rotator. 



In Ceratoclus,ii^ in all limb-bearing vertebrates, the nerves which 

 supply the limbs converge from an extent of the spinal cord 

 which is great compared with the breadth of attachment of the 

 limb. In Gcratodus the pectoral fin for example is supplied by a 

 single nerve-trunk to which fibres from four spinal nerves con- 

 tribute. After passing the axil of the limb this main trunk divides 

 into two, one of which turns round the anterior border of the fin 

 to the dorsal surface, while the other runs straight onwards 

 along the middle of the ventral surface. 



If the primordial limb consisted of a wide fold flanldug a 

 number of vertebral segments, as seems highly probable from 



