i8 David Starr Jordan 



The fin rays of the vertical fins are little developed and very 

 numerous, both being primitive characters. 



The Sharks. —The sharks and skates show likewise a 

 very large number of vertebrae, 120 to 150 in the species in 

 which they have been counted. In these fishes no compara- 

 tive study of the vertebrae has been made. The group is a 

 very ancient one in geological time, and in the comparatively 

 few remaining members of the group, the vertebrae, in fact 

 the entire skeleton, is in a very primitive condition, the ver- 

 tebrae being cartilaginous, the fin rays slender and very nu- 

 merous, not provided with .separate intenspinal bones. The 

 sharks are free-swimming fishes, and with them as with the 

 eels, flexibility of body is essential to the life they lead. 



One of the living sharks, Chlamydoselachiis, said to be the 

 oldest living type of vertebrate, has the body greatly elon- 

 gate, fairly eel-shaped, and it doubtless has a maximum num- 

 ber of vertebrae. A large number of cartilaginous vertebrae 

 is also found in the group of Chima;ras , and in the Dipnoi, a 

 very ancient type allied to the ganoids, and doubtless the 

 parent stock of the batrachians and through these of the 

 reptiles, birds, and mammals. Among the batrachians a 

 reduction in the number of vertebrae is associated with the 

 abandonment of aquatic life. 



Ganoid Fishes. — It may be taken for granted that the an- 

 cestry of the various modern t3'pes of bony fishes is to be 

 sought among the ganoids. All the fossil forms in this 

 group have a notably large number of vertebrae. The few 

 now living are nearly all fresh-water fishes, and among these, 

 so far as known, the numbers range from 65 to no.* 



Soft-rayed Fishes. — Among the Teleostei or \>o\\y fishes, 

 those which first appear in geological history are the Isospon- 

 dyli, the allies of the salmon and herring. These have all 

 numerous vertebrae, small in size, and none of them in anj^ 

 notable degree modified f or specialized. In the northern 

 seas Isospondyli still exceed all other fishes in number of 



* Sixty -seven in Polypterus, no in Calamoichthys, 95 in Amia, etc. 

 t As is indicated by the name Isospondyli, from icros, equal, ctttov- 

 §vXos, vertebra. 



