! 60 TELE OS TOMES 



as aggressive as sharks. They are remarkably tenacious 

 of life, and their complete armouring of dermal plates 

 renders them practically invulnerable. 



In development Lepidosteus has apparently more prim- 

 itive features than Acipenser (v., p. 207 ; also Jour, of 

 Morph. XI, No. i). 



Of all recent Ganoids, Lepidosteus must certainly be 

 looked upon as retaining most perfectly the structural 

 Characters of the most abundant and probably the most 

 generalized Palaeozoic and Mesozoic forms. Its genus, it 

 is true, is not known to occur earlier than the Eocene, but 

 its structures scales, fins, labyrinthine teeth and partially 

 calcified skeleton are known to have been possessed, 



Fig. 165. The sturgeon, Acipenser sturio, L. X jV- Streams entering North 

 Atlantic. (After GOODE in U. S. F. C.) 



even in their details, by a number of the older genera and 

 families. 



The Sturgeons, Acipenser > Scaphirhynchus , Psephurus* 

 Polyodon, must in many ways be looked upon as of a highly 

 adaptive or even retrogressive character. There is strong 

 evidence that in their descent a large proportion, and, in- 

 cases, all of their dermal armouring has been lost, and that 

 their cusp-like ancestral teeth have either disappeared or 

 are retained in a rudimentary condition. 



The interrelationships of the four surviving forms of 

 sturgeons have not as yet been definitely suggested ; transi- 

 tional fossil forms have thus far been lacking, and the 

 relative importance of the different structures in the recent 



