PISCES FISHES. 493 



Jin, forms the tail ; and a third, likewise situated in the median 

 line at a short distance behind the anal orifice, is called the anal 

 Jin from that circumstance. 



These fins present two sets of bones : the interspinous bones, 

 which form the basis to which they are affixed ; and (he Jin-rays. 



The interspinous bones (Jig. 21. 74) form a series of strong 

 dagger-like bones, deeply implanted in the flesh along the mesial 

 line of the body, between the two great masses of lateral muscles : 

 their points generally penetrate to a little distance between the 

 spinous processes of the Vertebrse, to which they are connected by a 

 ligamentous attachment ; whilst to their opposite extremity, which 

 may be compared to the hilt of the dagger, the corresponding fin- 

 rays are affixed by a beautiful articulation. There is generally 

 only one interspinous bone affixed to a vertebral spinous process, 

 but in the Flat-fishes (Pleuronectida) there are two ; and, more- 

 over, in that remarkable family, the, inferior spinous process of 

 the first caudal vertebra, which j as we have already seen, is of 

 enormous size, frequently has not fewer than six or seven interspi- 

 nous bones appended to its extremity. 



Each interspinous bone consists of two pieces united by a 

 suture ; one portion representing the blade, the other the handle 

 of the dagger, to which we have compared it. 



The fin-rays of fishes are of two kinds, being either solid and 

 apparently composed of one strong piece, like those which sup- 

 port the anterior half of the dorsal fin of the Perch (Jig. 75), in 

 which case they are called spinous rays ; or else they are composed 

 of several slender stems derived from one common root, every one 

 of which is made up of numerous pieces : these, which bear the 

 name of soft rays, are found in the posterior portions both of the 

 dorsal and anal fin of the perch, and are invariably met with in 

 the tail of all fishes possessed of a caudal fin. This difference in 

 the structure of the fin-rays, trivial as it might appear, is a circum- 

 stance to which much importance is attached by icthyologists, who 

 hence derive the means of separating osseous fishes into two great 

 groups, the Acanthopterygii, or such as possess spinous rays in 

 the composition of their dorsal fin ; and the Malacopterygii, in 

 which all the fin-rays are soft. Every fin-ray, whether spinous or 

 soft, is in reality made up of two lateral halves placed side by side : 

 in the soft rays these are easily separable ; but in the spinous rays 

 they are firmly united along the median line, so as to represent but 

 one bone. 



