Beyond the Horizon 26() 



beach, and are popularly known as "mermald's-purses." 

 Near the head of the embryo a slit in tlie case allows 

 the water to enter for its respiration, the water sub- 

 sequently passing out through another slit at the oppo- 

 site end. On issuing from its cradle the young shark 

 ruptures the end near its head, and carries with it a 

 yolk bag which it retains for nourishment until such 

 time as it is able to seek food. Its respiration at this 

 stage of its life is performed by the aid of filaments 

 projecting from the gills through the gill clefts. As it 

 gets older, which is to say, by the time it uses its teeth 

 efficiently, these filaments and the yolk bag disappear. 

 What in most fishes which are familiar to us is a 

 calcified frame — that is to say, a skeleton of bone — is 

 in the sharks, a tough, or cartilaginous, structure. This 

 shows that they are of a quite primitive nature, that 

 they had their beginning long before bones were in 

 evidence. But their claim to ancient ancestry is estab- 

 lished in other ways than this; their remains are pre- 

 served in rocks much older than those which hold the 

 fossils of bony fishes. From the abundance of teeth 

 which are found in the older deposits, it is quite prob- 

 able that the sharks were much more numerous in 

 former times than they are to-day. They were also 

 much larger; certain individuals of one genus {Carchar- 

 odon) being over ninety feet in length. There is 

 one existing species of this genus (C rondeletii) the 

 members of which are thirty feet long, but it is nearly 

 extinct. The abundance of sharks' teeth is so great, in 

 fact, that some of the beds in which they occur are 

 quarried to obtain the fossil remains for fertilizing 

 purposes. 



