324 AN1MTJAL REPORT SMITHSONIAN INSTITUTION, 194 



state, and by the heavy bony armature of the head. Many were not 

 very different from the primitive sharklike ancestor, since their inner 

 skeleton consisted at least partly of cartilage or gristle. They were 

 covered by heavy scales, the outer surface of which was ornamented 

 by an enamellike substance known as ganoin. Many of them pos- 

 sessed most peculiar tails, known as gephyrocercal, which are really 

 two tails in one, the extreme tip being a remnant of the original 

 true tail which degenerated. Their pectoral and pelvic fins had 

 developed so as to be very like limbs. It had been supposed that the 



MAN 



4.00.000.000 B.C 



PRIMITIVE ANCESTRAL FI5HES 



Figure 1.- 



-Diagram illustrating the main lines of the evolution of fishes belonging to the 

 groups Selachians and Pisces. 



sole living representatives of this ancient line were the few rather 

 scarce species of "lungfishes" living in fresh water in America, 

 Australia, and Africa. They are degenerate forms which are but 

 feeble shadows of tlieir active and predaceous ancestors. 



After having lived and flourished for some 250 million years all 

 those other numerous and vigorous Crossopterygian fishes had been 

 supposed to have become extinct by 50 million years ago. The 

 record in the rocks showed how, after having occurred in great num- 



