THE MISSISSIPPIAN PERIOD. 535 



the minor stages of their progress are less significant than the general 

 ones. They are therefore discussed here for the period as a whole. 



Fish appear to have first effectually invaded the open sea in the 

 Devonian period, but during that period true marine fishes seem to 

 have been inferior, in number and variety, to those of the inland 

 waters. But by the middle of the Mississippian period the marine 

 fishes had made such relative progress that they were in unquestioned 

 supremacy, while the fresh-water forms had notably declined, if we 

 may trust the record. The extension of the epicontinental seas, and 

 the consequent reduction of the land-areas, and doubtless the land- 

 waters, favored the former and restricted the latter. In the seas, 

 the supremacy of the sharks was almost uncontested. The cephalo- 

 pods, probably the master forms among the invertebrates, were no 

 match for the newcomers and, as already noted, played a very small 

 part in the life of the times. The great sea reptiles and sea mammals, 

 which later preyed upon and kept in subjection the sea fishes, had 

 not yet come to dispute their sway. Under these conditions, and in 

 a new, rich, and expanding realm, it is not surprising that the fish of 

 the period had a really wonderful development. They were appar- 

 ently much more abundant than in any later period; at least this is 

 true of the sharks. Up to 1889, about 400 species, nearly all elasmo- 

 branchs, had been reported from the Mississippian formations of 

 America, and about 200 additional species from Europe. 1 



The relics are chiefly teeth, spines, and dermal ossicles, and per- 

 haps the number of species is exaggerated by duplicate naming, though 

 this is doubtless much more than offset by the unrecorded forms. 

 The elasmobranchs held undisputed precedence, and of these at least 

 three-fourths had crushing or pavement teeth, adapted to breaking 

 the shells of mollusks and crustaceans, and the trituration of seaweeds. 2 

 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 to prevent the slipping of the prey. The multiplication 

 of these chonchivorous sharks introduced a new factor into the inver- 

 tebrate struggle for existence, and perhaps accounts for the decline 

 or disappearance of some forms and the modification of others. 



1 Newberry, Pal. Fishes, N. Am. Mon. XVI, U. S. Geol. Surv., 1889, pp. 182-83. 

 'Newberry, ibid., p. 184. 



