80 PALEOZOIC FISHES OF NORTH AMERICA. 
dable in our present seas; and in the Tertiary the great Carcharodon megal- 
odon was perhaps the most terrible of all the carnivorous inhabitants of the 
sea, yet it was apparently unarmed, except by the huge lancet-shaped teeth, 
as large as one’s hand, with which its cavernous mouth was thickly set. 
Like nearly all Sharks of modern date, it was without defensive fin-spines, 
while in the Carboniferous seas hundreds of species bore these defenses, 
which show an almost infinite series of modifications for securing great 
effectiveness. Even in the Upper Silurian the first of the Sharks, Onchus, 
seems to have been provided with these organs, and in the Devonian and 
Carboniferous ages the fashion was adopted by many other fishes; the 
Acanthodians, for example, bristled with spines, and from this characteristic 
have taken their name. So the Placoderms sometimes added to the nega- 
tive defenses of their plate armor pectoral and perhaps dorsal spines, which 
must have assisted in repelling their enemies. 
It may be said that we have no information in regard to the number 
of defenseless Sharks in the Carboniferous sea, but if such Sharks could 
leave no spines as proof of their existence they must have been provided with 
the other bony organs of the Elasmobranchs—teeth and dermal tubercles. 
These we find abundantly in the ancient sediments, but when we group 
them in genera and species, we see that the variety of dermal tubercles is 
far surpassed and that of the teeth nearly equaled by the spines. Hence 
we must conclude that most of the Paleozoic Sharks were provided with 
_ spines. 
In regard to the influences which have operated to produce the general 
disarmament which we find recorded in the life history of the Sharks we can 
at least imagine a solution of the problem. In my judgment it is simply 
the supersedure of a useful device by one still more useful; that is, the 
substitution of attack for defense, of activity and intelligence for mere nega- 
tive resistance, a change in the mode of warfare, and the disappearance by 
disuse and atrophy of devices which had become useless and obsolete. I 
have elsewhere’ incidentally referred to this question, and have there used 
the following language, which I venture to quote: 
‘ Paleontology of Ohio, vol. 1, p. 266. 
