160 TELEOSTOMES 
as aggressive as sharks. They are remarkably tenacious My 
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, X1, No. 1). 
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 Palzozoic 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 yy. 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 t3 
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 
