618 GEOLOGY 



progress that they were in unquestioned supremacy, while the 

 fresh-water forms had declined notably, so far as the record shows. 

 The extension of the epicontinental seas, and the consequent reduc- 

 tion of the land-areas, and doubtless of the land-waters, favored 

 the former and restricted the latter. In the seas the supremacy 

 of the sharks was almost uncontested. They were more abundant, 

 apparently, than in any later period. Up to 18cS9, about 400 species 

 had been reported from the Mississippian formations of America, 

 and about 200 additional species from Europe. 1 The fossils are 

 chiefly teeth, spines, and dermal ossicles. Three-fourths of the 

 species had crushing or pavement teeth, adapted to breaking the 

 shells of mollusks and crustaceans, and the trituration of seaweeds. 

 The tooth-pavement was formed of large plates of thicknesses 

 ranging up to one and one-half inches, composed of solid dentine 

 below and a thick sheet of enamel above, which was pitted, ridged, 

 or otherwise roughened. The number of spines preserved is ex- 

 ceptionally great when compared with the teeth and dermal 

 ossicles, and implies that the spines were more numerous than in 

 later times. The subsequent loss of spines, like the loss of the plates 

 and other clumsy defensive devices, was compensated for by a gain 

 in agility, intelligence, and more effective weapons of attack. But 

 the great development of species at this time doubtless had its 

 special reason in the fact that most of the sharks were provided 

 with pavement teeth, which were ineffectual weapons against 

 other sharks armed with piercing and cutting teeth. The spin;v< 

 of the one group were therefore a defense against their aggressive 

 kin. The arthrodirans and lung fishes had declined, as compared 

 with the Devonian period. 



Of the fishes frequenting the inland and coastal waters, probably 

 the culminating type was of the order to which the modern ^arpike 

 belongs. The curious tribe of ostracoderms (p. 590) had nearly 

 or quite disappeared. 



The fishes probably made up the whole, or nearly the whole, 

 of the vertebrate fauna of the seas. The ostracoderms may have 

 entered the sea, but they were probably fresh-water forms in ihe 



1 Newhorry, Paleozoic Fishes of North America, Monogr. XVI, U. S. Geol. 

 Surv., 1889. 



