FISH-EEMAINS OF THE CARBONIFEEOUS EOCKS. 81 



armed with a great number of teeth. In some cases the 

 teeth are produced into sharp and pointed blades, whicli 

 serve for holding slippery prey ; in other cases they are 

 blunt, rounded or flattened, and serve for crushing hard 

 material, such as shells or coral. Each jaw, upper and lower, 

 contains several rows of teeth, which are frequently arranged, 

 not longitudinally, but in rows that run transversely and 

 somewhat obliquely. The teeth are throughout life con- 

 tinually shed and as continually replaced. The new teeth 

 are developed in a fold running along the inside of the 

 jaw, and the whole pavement of the jaw with its numerous, 

 rows moves forward over the jaw to the outer edge, where 

 they are torn away or shed. To allow of this process the 

 teeth are not set in sockets, but merely imbedded in the 

 membrane that covers the jaw. The fact that in different 

 parts of the jaw of the same fish, as in the Port Jackson 

 shark, teeth of both kinds, pointed and crushing, are devel- 

 oped, and that in the same jaw the numerous teeth are of 

 different sizes and shapes, is one that must be borne in mind 

 in dealing with the separate teeth that are found in a fossil 

 condition. The tail of the shark-like plagiostome fishes is not 

 externally symmetrical, like that of the herring or salmon, but 

 unsymmetrical (heterocercal), the upper lobe being produced. 

 In the rays the tail is often greatly elongated. There are 

 two pairs of lateral fins, of which the anterior pair are the 

 larger, and are in the rays extended so as to form the flat- 

 tened wing-like sides of the fish. There are several gill-slits 

 on each side of the head, and these are not covered over with 

 a gill-flap, or opuculum, such as is seen in our commoner 

 fishes which belong to quite a different group — that of the 

 Teleosteans. The scales are peculiar. They consist as a 

 rule of smaller or larger separate plates which bear sharp 

 tooth-like points composed of tooth-like substance. In the 



