518 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



found that the changes do not affect the fauna as a whole, but that they 

 are almost entirely confined to classes comprising forms whose migration 

 from place to place is more or less completely dependent on marine cur- 

 rents. 



Among Paleozoic faunas nearly all of the species so limited belong to 

 the lithisdid sponges, the reef forming corals and the bryozoa, most of 

 which begin with a free-swimming larval stage but later on assume a 

 sessile habit of growth. In that they depend on marine currents for the 

 extension of their ranges, these organisms resemble the planktonic or 

 pelagic species, but in their mature conditions they are very different. 

 Thus, while the truly pelagic forms breed and pass their existence near 

 the surface of the water and are quite indifferent to its depth and to the 

 character of the bottom beneath them — features insuring extraordinary 

 extension of range — the corals and other things immediately under con- 

 sideration require limited depths of water and certain bottom conditions 

 before their larvae can secure a suitable foothold and develop mature 

 characteristics. Although the geographical range of such species in con- 

 tinental seas is commonly less than that of the mollusks, Crustacea, and 

 brachiopods, it is, on the contrary, greater where, as between different 

 continents, it is necessary to cross relatively wide and deep seaways which 

 would more effectually bar the distribution of the other classes. 



The diminution and perhaps final extinction of sponges, corals, and 

 bryozoa away from the inlet, as northeastward from the Mississippi em- 

 bayment in the case of the upper Stones Eiver, the Lowville, and the 

 Onondaga, especially in view of the fact that a similar diminution is more' 

 or less clearly manifested by all the faunas invading from the Gulf of 

 Mexico, can not be wholly attributed to increasing unfavorableness of 

 environment. Unfavorable conditions of bottom doubtless are responsible 

 for much of the irregularity of distribution, but not for its final extent. 

 This depends on whether there is a current capable of carrying the free 

 larvae past the unfavored to more propitious areas. 



It is thus that the absence in the Pamelia in New York of the numer- 

 ous sponges, corals, and bryozoa found in the corresponding parts of the 

 Stones River group in Tennessee is explained. Also the slower elimina- 

 tion of these classes in the Lowville fauna northward from middle Ten- 

 nessee. 



Regarding these current-transported mero-planktonic constituents of 

 faunas, it is to be further noted that within the same continental sea their 

 occurrence in deposits perhaps hundreds or even thousands of miles apart 

 is a fact deserving the highest consideration. It is therefore with implicit 



