98 The American Geologist. August, ism 
with in>n to such n degree thai though well nigh impenetrable 
they arc in danger from that which should be their protection, 
so Dinichthys and his fellows were so thickly encased in bone 
that it is not easy to sec how they can have possessed propor- 
tionate activity. The next step in naval architecture will 
probably be to follow the example set by nature in post-Devo- 
nian da}-s and strip off or reduce the ponderous armour-plates, 
as she has stripped off or thinned down the bony shields of 
her mail-clad fishes. With increase of brain and nerve there 
has come a diminution of plate and scale so that the swift 
Teleosts of the recent seas have steadily superseded the huge 
Ganoids and Placoderms of old. 
Nature, though slow, learned her lesson millions of years 
ago; man has his yet to learn. About the close of the Devo- 
nian age she began to discontinue the building of the enormous 
placoderms which we have been describing, and adopted a 
lighter form — the Ganoids — of which the American Gars may 
be considered a surviving pattern, and being apparently 
pleased with the result of her experiment she took a further 
step at the commencement of the Cretaceous era and covered 
her fleets with thin scales of horn, transferring the bones to 
their inner parts and thus introducing her bony fishes — the 
Teleosts — her latest type of fish-construction. These trust to 
activity and watchfulness rather than to armour for safety. 
It is singular, or at least unexpected, that the armour of 
these fishes was "all forward." Their hinder parts were cer- 
tainly in many, and probably in all cases, naked or scaly. Had 
they no dangers astern from which they needed protection V 
Or Mere the only creatures that they coidd fear so slow that a 
stern-chase was always a losing race? We cannot tell. It 
says little for the speed of the pursuer when the pursued leaves 
his back undefended. 
Monsters such as these can scarcely have been mud-fishes. 
buried in the bottom with merely head and jaws exposed. 
Teeth like theirs can not have been evolved except in crea- 
tures that employed them in open struggle with their prey or 
their foes. They are scarcely the weapon of cunning or con- 
cealment. 
One of the two views seems necessary: Either they had no 
foes to fear behind or they kept their soft back parts out of 
