THE EARLIEST KNOWN VERTEBRATES 21 



they declined in numbers even during the Carbonifer- 

 ous Period, and later dwindled almost to extinction. 

 And while sharks again increased, they never reached 

 their former abundance, and the species that arose 

 were swift, predatory forms, better fitted for the struggle 

 for existence. 



REFERENCES 



The early fishes make but little show in a museum, both on 

 account of their small size and the conditions under which they 

 have been preserved. The American Museum of Natural 

 History has a fine exhibition series of these old fishes; the Mu- 

 seum of Comparative Zoology has a large collection of them and 

 there is a considerable number of fine teeth and spines of Carbonif- 

 erous sharks in the United States National Museum. 



Hugh Miller's "The Old Red Sandstone" contains some 

 charming descriptions of his discoveries of Pterichthys and re- 

 lated forms, and this book will ever remain a classic. 



Pterichthys the Wing Fish 



