40 



in the intervals between the spines, hence the name of these bones. There are usually 

 two interspinous bones between two adjacent vertebral spines, never more than two, 

 but in many cases only one : one interspinous bone, in Fig. 1 on PL X, has two shafts 

 to a single head, the end of a vertebral spine lying between the two shafts. In the 

 middle region of the body each interspinous bone has a small wing-like expansion on 

 each side of the shaft just below the head. On the dorsal side the interspinous bones 

 are perpendicular to the vertebral column at about one-fourth of the whole length 

 from the anterior end of the body : from this region to the tail they slope more and 

 more backwards ; from this region forwards they slope more and more forwards. In 

 front of the dorsal spine of the second vertebra there are three short stout interspinous 

 bones whose inner ends lie close to the middle dorsal line of the cranial portion of the 

 skull. In front of these is a large curved spine-shaped bone pointed at its outer end, 

 stout and blunt at its inner, which curves forwards almost parallel to the axis of the 

 skull. To the dorsal side of this bone are attached two short interspinous bones. 



On the ventral side the interspinous bone in front of the ventral spine of the eleventh 

 vertebra is almost perpendicular to the vertebral column : from this point to the tail 

 region the interspinous bones slope more and more towards the posterior end. In 

 front of the same interspinous bone there is one other free inters^Dinous bone slightly 

 inclined forwards, and in front of this is a stout curved cylindrical bone, wliich 

 terminates the series anteriorl}- and forms the posterior boundary in the median plane 

 of the main body-cavity of the fish. This cylindrical bone bears four shortened 

 interspinous bones which are inclined forwards and are successively shorter and shorter, 

 the most anterior being merely a small head without a shaft. 



The fin-rays of the dorsal and ventral median fins, of the dorsal fin and anal fin as 

 they are usually termed, are not articulated directly with the outer ends or " heads " 

 of the interspinous bones : to these heads are attached a series of nodules of cartilage, 

 elongated from before backwards, and somewhat cylindrical in shape. Except in a 

 certain region each of these nodules is situated between two adjacent heads of 

 interspinous bones, attached to both of them. The region excepted is the anterior 

 part of the dorsal series, where one nodule is attached separately to each head. 



The fin-rays have the following structure. Each ray is compound and constructed 

 somewhat in the same fashion as a vertebral spine. It consists of a right and left half, 

 which are separate and divergent at their inner ends, attached together in the median 

 plane of the fish for the outer seven-eighths of their length. The divergent ends embrace 

 one of the cartilaginous nodules previously described, bestriding it as a man bestrides a 

 saddle, and are attached to it by fibrous membrane in this position. One of the halves 

 of the fin-ray thus belongs to the right or coloured side of the fin, the other to the left 

 or white side. In consequence of the mode of attachment described, the fin-ray can 

 only move on the cartilaginous nodule backwards and forwards : as a matter of fact 

 its motion is limited between a position in which it is perpendicular to the longitudinal 

 axis of the fish, and a position in which it lies inclined backwards from its attached 



