FOSSIL FISHES. 



11 



embryonic tail is the primitive type from which the ancient hetero- 



cercal and the more modenr homocercal forms have been developed. 



As the skeleton in all the earlier Sharks was cartilaginous like 



Fig. S.— Fort-Jackson Shark, Cestracion galeatus, Australia (recent), stf, Anterior dorsal 



spine ; sp", posterior dorsal spine. 



their modern representatives, and in many of them even noto- 

 chordal, it is not usually preserved ; there is often therefore great 

 difficulty in obtaining satisfactory evidence for the correct deter- 



Fig. 9. 



Fig. 10. 



-J 



Fio. 0.— Posterior spine of Cestracion (recent) from Japan. 



Fig. 10.— Spine of Chima ra monstrosa, Linn. (recent), Norway, showing the broadly-expanded 

 base of spine (/>), which is inserted in the muscular tissue. 



mination of some of these older fishes in a fossil state. Thus in 

 the great majority of instances we have only the detached spines, 

 teeth, and shagreen left, all else has disappeared ;* but in the 



* In consequence of this, many genera of fossil sharks and very many species 

 are based solely upon detached spines or teeth, whilst of the rest of the fish we 

 know absolutely nothing. 



