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ancient sharks. In the Corniferous limestone, we find the 
fossils described under the name of M/acheracanthus. These 
were fin-spines of fishes, but were unlike anything known 
among modern sharks, being “ double bayonets ” in structure, 
of great size, and constituting most formidable side-arms. At 
present, but few sharks, and those of small size, have spined 
fins. In the Huron shale, equivalent to a part of the Portage 
group, the fossils known as Ctenacanthus appear; these were 
dorsal spines a foot long. As the complement of all this 
system of weapons among the sharks, the ganoid fishes of the 
same period had an amount of bony encasement that was well 
adapted to the necessities of their circumstances. It would 
seem, however, as though there must have have been numer- 
ous less protected fishes, probably of smaller size, which have 
left but few traces in the rocks, but which served as food to 
these mail-clad and sword-spined monsters of the period. 
In coming down to latter ages, we find a general disarma- 
ment among the sharks, and a consequent, or at least a cor- 
responding, disappearance of the heavy armor worn as a de- 
fence by the ganoids. Our living fishes certainly have noth- 
ing of spiny side-arms, or bony encasements. 
An interesting question arises in reviewing the history of 
these remarkable forms of fish-life in the Devonian age. 
When and how was all this offensive and defensive arma- 
ment acquired? Are we to look to direct endowment, or to 
inheritance, as the source whence it was derived? Of the 
latter, the evidence as yet is wanting; and the inquiry re- 
mains, as one of no slight interest. A similar problem is 
presented by much of the ornamentation of the lower ani- 
mals. ‘The minute and elegant chasing upon so many ganoid 
scales, the ornamental shields of so many trilobites, the 
evident beauty that must have belonged to much of the 
Palzeozoic shell-fauna, all suggest this query; and it becomes 
still more marked in considering the exquisite sculpturing of 
the Upper Silurian crinoids and eystideans, and the beauty of 
color and form in modern polyps and both modern and 
ancient echinoids ; in all of which groups the explanation of 
sexual selection, advanced in regard to higher animals, has 
